THE  INTERNATIONAL  SCIENTIFIC  SERIES. 

VOLUME  XIV. 


THE  INTERNATIONAL   SCIENTIFIC   SERIES. 

Works  already  Published. 

I.  THE  FORMS  OF  WATER  IN  RAIN  AND  RIVERS,  ICE  AND 
GLACIERS.  By  J.  TYNDALL,  LL.  D.,  F.  R.  S.  With  26  Illustrations. 
Price,  $1.50. 

II.  PHYSICS  AND  POLITICS;  OR,  THOUGHTS  ON  THE  APPLICATION  OF 
THE  PRINCIPLES  OF  "NATURAL  SELECTION"  AND  "INHERITANCE"  TO 
POLITICAL  SOCIETY.  By  WALTER  BAGEHOT.  Price,  $1.50. 

III.  FOODS.    By  DR.  EDWARD  SMITH.     Illustrated.     Price,  $1.75. 

IV.  MIND    AND    BODY:    THE   THEORIES   OF   THEIR   RELATIONS.      By 

ALEXANDER  BAIN,  LL.  D.     Price,  $1.50. 

V/  THE  STUDY  OF  SOCIOLOGY.    By  HERBERT  SPENCER.    Price,  $1.50. 
VI.    THE  NEW  CHEMISTRY.     By  PROFESSOR  JOSIAH  P.  COOKE,  of  Har- 
vard University.     Illustrated.    Price,  $2.00. 
VII.    ON  THE  CONSERVATION  OF  ENERGY.    By  PROFESSOR  BALFOUR 

STEWART.     Fourteen  Engravings.     Price,  $1.50. 

VIII.    ANIMAL  LOCOMOTION;    OR,  WALKING,   SMIMMING,   AKD  FLYING. 
By  DR.  J.  B.  PETTIGREW,  M.  D.,  F.  R.  S.     119  Illustrations.     Price, 
$1.75- 
IX.    RESPONSIBILITY    IN    MENTAL    DISEASE.      By   DR.    HENRY 

MAUDSLEY.    Price,  $1.50. 
X.    THE  SCIENCE  OF  LAW.     By  PROFESSOR  SHELDON  AMOS.     Price, 

$1-75. 

XL    ANIMAL   MECHANISM;    OR,   AERIAL  AND  TERRESTRIAL  LOCOMO- 
TION.    By  C.  J.  MAREY,  Professor  of  the  College  of  France,  Member  of 
the  Academy  of  Medicine,  Paris.     117  Engravings.     Price,  $1.75. 
XII.    HISTORY   OF   THE   CONFLICT    BETWEEN   RELIGION   AND 
SCIENCE.     By  JOHN  W.  DRAPER,  M.  D.,  LL.  D.     Price,  $1.75. 

XIII.  THE  DOCTRINE  OF  DESCENT  AND  DARWINISM.    By  OSCAR 

SCHMIDT,  Professor  in  the  University  of  Strasburg.     Price,  $1.50. 

XIV.  THE  CHEMISTRY  OF  LIGHT  AND  PHOTOGRAPHY;    IN  ITS 

APPLICATION  TO  ART,  SCIENCE,  AND   INDUSTRY.     By  DR.  HERMANN 
VOGEL.     One  Hundred  Illustrations.     (In press.) 

XV.    FUNGI ;    THEIR  NATURE,  INFLUENCE,  AND  USES.     By  M.  C.  COOKE, 
M.  A.,  LL.  D.     Edited  by  REV.  M.  J.  BERKELEY,  M.  A.,  F.  L.  S.    With 
109  Illustrations.     (In  press.} 
XVI.     OPTICS.     By  PROFESSOR  LOMMEL,  University  of  Erlangen.     (In  press.) 


PLATE  I. 


Fron  tispiece. 


Wcodbury  process. 


PHOTOGRAPH  OF  THE  MOON, 

(after   RUTHERFORD'S   original   negative.) 


Seep.  193- 


THE   INTERNATIONAL   SCIENTIFIC   SERIES. 


THE 


CHEMISTRY  OF  LIGHT 


AND 


PHOTOGRAPHY. 


BY 


DR.   HERMANN  VOGEL, 

PROFESSOR  IN   THE  ROYAL  INDUSTRIAL    ACADEMY  OF   BERLIN. 


WITH  ONE  HUNDRED  ILL 


NEW  YORK: 
D.    APPLETON    AND    COMPANY, 

549    AND    551    BROADWAY. 
1875. 


PBEFACE. 


AMONG  the  splendid  scientific  inventions  of  this 
century,  two  are  specially  prominent — Photography 
and  Spectrum  Analyst.  Both  belong  to  the  province 
of  Optics,  and  at  the  same  time  of  Chemistry.  While 
Spectrum  Analysis  has,  down  to  the  present  time, 
remained  almost  exclusively  in  the  hands  of  the  learned, 
Photography  passed  immediately  into  practical  life, 
spread  over  almost  every  branch  of  human  effort  and 
knowledge,  and  now  there  is  scarcely  a  single  field  in 
the  universe  of  visible  phenomena  where  its  productive 
influence  is  not  felt. 

(li  brings  before  us  faithful  pictures  of  remote  regions, 
of  strange  forms  of  stratification,  of  fauna,  and  of  flora  ; 
it  fixes  the  transient  appearances  of  solar  eclipses ;  it 
is  of  great  utility  to  the  astronomer  and  geographer  ; 
it  registers  the  movements  of  the  barometer  and 
thermometer;  it  has  found  an  alliance  with  porce- 
lain painting,  with  lithography,  metal  and  book  typo- 
graphy; it  makes  the  noblest  works  of  art  accessible 


VI  PREFACE. 

to  those  of  slender  means.  It  may  thus  be  compared 
to  the  art  of  printing,  which  confers  the  greatest  benefit 
by  multiplying  the  production  of  thought,  for  it  conveys 
an  analogous  advantage  by  fixing  and  multiplying 
phenomena.  But  it  does  more  than  this.  A  new  science 
has  been  called  into  being  by  Photography,  the  Chemis- 
try of  Light;  it  has  given  new  conclusions  respecting 
the  operations  of  the  vibrating  ether  of  light.^  It  is 
true  that  these  services,  rendered  by  Photography  to 
art  and  science,  are  only  appreciated  by  the  few.  Men 
of  science  have  in  great  measure  neglected  this  branch 
after  the  first  enthusiasm  excited  by  Daguerre's  inven- 
tion had  evaporated ;  it  is  only  cursorily  that  physical 
and  chemical  matters  are  treated  on  in  manuals  of 
Photography. 

Taking  this  into  consideration,  it  has  seemed  expedient 
to  the  Author  to  give  a  popular  view  of  Photography  and 
the  Chemistry  of  Light,  showing  their  important  bearing 
on  science,  art,  and  industry.  The  Publisher  has  met 
the  Author  in  the  readiest  manner,  not  only  by  providing 
numerous  woodcuts  to  explain  the  text,  but  by  obtaining 
specimens  of  the  latest  discoveries  in  Photography,  at  a 
considerable  cost.  So  that  as  the  tables  annexed  give  a 
view  of  what  is  achieved  by  Photography  in  connection 
with  typography,  he  trusts  the  work  may  meet  a  friendly 
reception. 

THE  AUTHOR. 

BERLIN,  January,  1874. 


TABLE    OF    CONTENTS. 


1.     Development  of  our  Photo -Chemical  Knowledge            ...  1 

II.    The  Daguerreotype'              ...             ...             ...             ...  14 

III.  Paper  Photography  and  the  Licht-paust  or  New  Talbot 

Process               ...             ...             ...             ...             ...  23 

IV.  The  Development  of  Modern  Photography      ...             ...  31 

V.     The  Negative  Process          ...             ...             ....            ...  37 

VI.     The  Positive  Process           ...             ...             ...             ...  46 

Light  as  a  Chemically- Operative  Agent           ...             ...  55 

Chemical  Effect  of  different  Sources  of  Light                ...  68 

IX.     On  the  Refraction  of  Light               ...             ...             ...  83 

X.     The  Photographic  Optical  Apparatus                ...             ...  89 

The  Chemical  Effects  of  Light           ...             ...             ...  105 

(a)  Operation  of  Light  on  the  Elements     ...             ...  107 

(I)  Chemical  Effect  of  Light  on  Salts  of  Silver        ...  109 

XII.     On  the  Correctness  of  Photographs    ...             ...             ...  120 

(a)  Influence  of  the  Individuality  of  the  Photographer  120 

(b)  Influence  of  the  Object,  of  the  Apparatus,  and  of 

the  Process             ...             ...             ...             ...  122 

XIII.  Light,  Shade,  and -Perspective            ...             ...             ...  134 

XIV.  The  Applications  of  Photography      ...             ...             ...  149 

§  I.  Portrait  Photography     ...             ...             ...  149 

§  II.  Landscape  Photography ...          .*...             ...  159 

§  III.    Phot ogramme try,   or    Levelling   by   Photo. 

graphy            ...              ...              166 

§  IV.  Astronomical  Photography             ,.,             ...  171 


Vlll  TABLE   OF   CONTENTS. 

CHAPTER 

XIY.     The  Applications  of  Photography, — continued. 

§  Y.  The  Photographic  Observation  of  Scientific 

Instruments 

§  VI.  Photography  with  reference  to  Medical  Re- 
search ...  ...  ...  ••• 

§  VII.  Photography  and  the  Microscope  ... 
§  VIII.  Microscopic    Photographs    and    the    Photo- 
graphic Pigeon  Post     ...  ... 

§  IX.  Pyro-photography 

§  X.  Magic  Photography 
§  XI.   Scamoni's  Heliographic  Process   ... 
§  XII.  Photography  and  Jurisprudence   ... 
§  XIII.  Photography,  Industry,  and  Art  ... 
XV.     Chromo -photography  ...  ...  ...  ... 

§  I.  Chromic  Combinations    ... 
§  II.  Heliography  with  Salts  of  Chromium 
§  III.  The  Production  of  Photo-reliefs    ... 
§  IV.  Printing  in  Belief 

§  V.  Pigment    Printing,    or    the    Production    of 
Charcoal  Pictures        ...  ... 

§  VI.  Light  Printing  ... 
§  VII.  Aniline  Printing 
§  VIII.  Photo-lithography 

§  IX.  Pyro-photography  with  Salts  of  Chromium... 
§  X.  Photography  and  the  Sand-blowing  Process 
§  XI.  The  Photometer  for  Chromo-Photography  ... 
§  XII.  The  Chemical  Effect  of  Light  and  the  Pea- 
Sausage         ...  ...  ...  ... 

XVI.     Iron,  Uranium,  and  Copper  Photography 
XVII.    The  Change  of  Glass  under  the  influence  of  Light 
XVIII.     Photography  in  Natural  Colours        ...  ... 

XIX.    Photography  as  a  Subject  to  be  Taught  in   Art   and 
Industrial  Schools  ...  ... 

Index 


LIST  OF  ILLUSTRATIONS. 


FIQ.  PAGE 

1.  Ivy  leaf       6 

2.  „         copied  on  stone,  produced  by  nitrate  of  silver    ...  6 

3.  4.     The  camera  obscnra           7,  8 

5.     Leaf  prints              ...         ...         ...         ...         ...         ...         ...  24 

6,7.     Copying  frames  for  Talbot  types 26 

8.  Flat  dish  containing  water           ...         ...         ...         ...         ...  27 

9.  Licht-paiis  leaf  prints              29 

10.  Negative  process  for  transporting  the  plate      ...         ...         ...  43 

11.  Copper  frame  to  press  sensitized  paper          ...         ...         ...  50 

12.  Masks  of  metal  or  cardboard       ...         ...         ...         ...         ...  53 

13.  Illustration  of  undulation       57 

14.  Kays  in  a  drop  of  water  ...         ...         ...         ...         ...         ...  61 

15,16.     Spectrum  illustrations      ...         ...         ...         ...         ...  02 

17.  Lines  in  the  spectrum       ...         ...         ...          ...         ...         ...  63 

18.  Solomon's  lamp             ...         ...         ...         ...         ...         ...  69 

19.  Drummond's  lime  light                 ...         ...         ...         ...         ...  70 

20.  Electric  light 71 

21.  Electric  light  cylinder                  72 

22.  Electric  battery            72 

23.  Electric  light  apparatus                ...         ...    .*    ...         ...         ...  73 

24.  Reflection  in  underground  places      ...         ...         ...         ...  74 

25.  Illustration  of  coloured  rays        76 

26.  The  earth                                            78 


X  LIST    OF   ILLUSTRATIONS. 

FIG.  PAGE 

27.  Kefracfcion         83 

28.  Angle  of  incidence            ...         ...         ...         ...         ...         ...  84 

29.  Angles  of  incidence     ...         ...         ...         ...         ...         ...  84 

30.  Triangular  prism.    ...         ...         ...         ...         ...         ...         ...  85 

31.  Deviations  of  rays        ...         ...         ...         ...         ...         ...  85 

32.  Parallel  rays           86 

33.  Earning  glass * 86 

34.  Focus           87 

35.  Focal  distance 87 

36.  Camera  obscura     .,.-       89 

37.  Camera 90 

38.  Focussing 91 

39.  Telescopic  lens 92 

40.  Magic  lantern         93 

41,42.     Solar  camera         ii;         95,96 

43, 44.     Stereoscope      ...  99 

45.  Burning  glass  ...         ...         ...         ...         ...         ...         ...  100 

46.  Lens 100 

47.  Prism  refraction           , 101 

48.  Stereoscopic  deflection      ...         ...         ...         .,.         ...         ...  101 

49.  American  stereoscope...         ...         ...         ...         ...         ...  103 

50.  Lenses  used  in  photography        ...         ...         ...         ...         ...  123 

51.  Distortnre  of  spheres 124 

52.  Cube  and  cylinder             ...         ...         ...         ...         ...         ...  134 

53.  Rectangles        ...  135 

54.  Pencils  of  light      137 

55.  Intersecting  angles      ...         ...         ...         ...         ...         ...  138 

56.  Abnormal  appearances  produced  by  divergent  lenses  ...         ...  139 

57.  Portrait  taking 140 

58 — 61.     Foreshortening  and  distance  143,  144 

62.  Distance  in  portraits  ...         145 

63—66.     Point  of  view  of  the  spectator        146,  147 

67.  Magnifying  apparatus             156 

63.  Apparatus  of  landscape  photographer  ...         162 


LIST   OF   ILLUSTRATIONS.  xi 

FIG.  PAGE 

69.  Revolving  camera        ...         ..«         ...         ...         ...         ...  165 

70.  Trigonometrical  surveying           ...         ...         ...         ...         ...  168 

71.  Measurement  of  altitudes  by  photography              170 

72.  Telescope  adapted  to  photography          ...         172 

73.  Telescope  parallactically  set  up         ...         ...         ...         ...  173 

74—76.     Eclipse  of  the  sun  at  Aden 182-184 

77.  Photographic  view  of  the  corona      185 

78.  Appendage  to  telescope  (Rutherford's)              187 

79.  Spectral  apparatus      ...         ...         ...         ...         ...         ...  195 

80.  Distance  of  heavenly  bodies        •••^r    •••         •••         •••         •••  196 

81.  Determining  of  angles  of  planets  by  photography            ...  198 

82.  Thermometer  and  barometer  reading  by  photographs 200 

83.  Photographic  instrument  for  ear  diagnosis 203 

84.  Heliopictor          204 

85.  Photographic  microscope        ...         ...         ...         ...         ...  206 

86.  Magnifying  powers            207 

87.  Ordinary  microscope 207 

88.  Photographic  camera  adapted  to  a  microscope...         ...         ...  208 

89.  Chromo-photography 223 

90.  Galvano-plastic  apparatus            -..  228 

91.  Pantograph       231 

92.  Gelatine  reliefs       232 

93.  Insoluble  films 234 

91  Photometer             262 


LIST  OF  PLATES. 


PLATE 

I.    Photograph  of  the  moon       ...         ...         ..,  ...       Frontispiece. 

II.    Negative  and  positive  of  the  licht-paus  process  To  face  page     96 

III.  Electrotype  from  heliograph            „             230 

IV.  Copper-plate  from  heliograph „             230 

V.     [Reduction  of  a  section  of  the  ordnance  survey 

map  of  Wales     ...         ...         ...         ...         ...  „             254 

YL    Effect  of  retouching  of  negatives „            £45 


THE 

CHEMISTEY  QF  LIGHT. 


CHAPTEE  I. 

DEVELOPMENT  OF  OUE  PHOTO- CHEMICAL  KNOWLEDGE. 

Diverse  Kinds  of  Operations  of  Light — Physical  and  Chemical  Changes — 
Bleaching  Effect  of  Light — Effects  of  Light  npon  Chloride  of  Silver 
and  Lunar  Caustic  (Nitrate  of  Silver) — Chemical  Ink — Pictures  upon 
Paper  saturated  with  Nitrate  of  Silver — The  Labours  of  Wedgwood 
and  Davy — The  Camera  Obscura — Niepce — Effects  of  Light  upon 
Asphaltum — Heliography — Its  Application  to  the  Production  of 
Paper  Money — Iodide  of  Silver  — Discovery  of  the  Daguerreotype. 

THE  light  which  radiates  from  the  great  central  body 
of  our  planetary  system  produces  manifold  effects  upon 
the  animate  and  inanimate  world,  some  of  which  are 
at  once  evident  to  the  senses,  and  have  been  known  for 
thousands  of  years,  while  others,  again/are  not  so  ap- 
parent to  the  eye,  and  have  been  discovered,  examined, 
and  utilized  only  through  the  observations  of  modern 
times. 


2  THE    CHEMISTRY   OF   LIGHT. 

The  first  effect  which  every  person,  however  unculti- 
vated, notices,  when,  after  the  darkness  of  night,  the 
sun  rises,  is  the  visibility  of  bodies.  The  rays  from 
the  source  of  light  are  thrown  back  (reflected)  from  these 
different  bodies,  they  reach  our  eyes,  and  produce  an 
impression  upon  the  retina,  the  result  of  which  is  the 
perception  of  material  objects  through  the  eye.  But 
soon  another  effect  is  observed,  not  through  the  eye,  but 
by  sensation.  The  sun's  rays  not  only  illumine  bodies 
upon  which  they  fall,  but  heat  them,  as  is  felt  when  the 
hand  is  held  in  the  rays.  Both  effects,  the  shining  or 
illuminating,  and  the  warming  effects  of  the  sunbeams, 
differ  very  essentially  from  each  other.  The  illuminat- 
ing effect  we  perceive  instantaneously ;  the  heating  effect 
is  only  felt  after  a  certain  time,  which  may  be  shorter 
or  longer,  as  the  heating  power  of  the  sun  is  stronger  or 
weaker. 

In  addition  to  these  two  effects  of  sunlight,  there  is  a 
third,  which  generally  requires  a  still  longer  period  to 
make  itself  noticed,  and  which  cannot  be  directly  per- 
ceived either  through  the  eye  or  by  sensation,  but  only 
through  the  peculiar  changes  which  light  produces  in  the 
material  wroiid.  These  are  the  chemical  effects  of  light. 

If  we  take  a  small  piece  of  wood,  and  bend  or  saw  it, 
we  change  its  form  ;  if  we  rub  it,  it  becomes  warm  ;  we 
change  its  temperature,  but  it  still  remains  wood.  These 
changes,  which  do  not  affect  the  substance  or  matter 
(stoff)  of  a  body,  we  term  physical. 

But,  if  we  set  fire  to  a  piece  of  wood,  strong-smelling 
gases  ascend,  ashes  are  deposited,  and  a  black  residuum 
remains,  which  is  totally  different  from  the  wood.  By 
this  process  a  quite  different  substance — charcoal — has 


DEVELOPMENT    OF    OUB   PHOTO-CHEMICAL    KNOWLEDGE.       3 

been  produced.  Material  changes  of  this  kind  we  term 
chemical  changes; — and  such  chemical  changes  are,  in 
an  especial  manner,  the  result  of  heat.  If,  for  instance, 
we  heat  a  bright  iron  wire  red  hot,  it  undergoes  ap- 
parently only  a  physical  (not  a  material  change).  But, 
if  we  allow  it  to  cool,  we  find  the  bright  rod  has  become 
dull  and  black ;  that  it  has  received  a  brittle,  black 
surface,  which,  in  the  process  of  bending,  easily  breaks, 
and  the  bright,  tough,  flexible  iron  has  undergone  a 
chemical  change.  That  is  to  say  ^af  change  of  substance 
has  taken  place,  the  iron  has  been  converted  into  another 
body,  into  black  oxide,  because  it  has  combined  with  a 
component  part  of  the  surrounding  air  with  oxygen. 

Chemical  changes  of  this  kind  are  not  only  produced 
by  heat,  but  also  by  light. 

It  has  long  been  known  that  when  the  colours  of  which 
fabrics  are  dyed  are  what  is  called  "not  fast  colours," 
they  fade  in  the  light,  that  is,  become  paler.  In  this 
case  the  coloured  material  changes  into  a  colourless 
or  differently  coloured  body ;  and  that  this  is  the  effect 
of  light  is  evident  from  the  fact  that  that  part  of  the 
material  in  question  which  is  covered  up  from  the  light 
— that  beneath  the  folds— "-remains  unchanged.  This 
discolouring  effect  of  light  has  been  long  turned  to  prac- 
tical use  in  the  bleaching  of  linen.  The  unbleached 
fabric  is  spread  out  in  the  sunlight,  and  repeatedly 
moistened  with  water ;  and  thus,  through  the  combined 
effect  of  light  and  moisture,  this  dark  colouring  sub- 
stance becomes  gradually  soluble,  and  *  can  then  be 
removed  from  the  linen  by  boiling  it  in  lie  (alkali). 

It  was  formerly  believed  that  the  changes  we  have 
just  described  were  caused  by  the  heat  which  is  pro- 


4  THE    CHEMISTRY   OF   LIGHT. 

duced  in  bodies  by  the  sun's  rays.  That  this  is  an 
erroneous  view  is  evident  from  the  fact  that  fabrics 
dyed  in  colours  which  are  "not  fast"  can  be  exposed 
for  months  together  in  the  temperature  of  a  hot  oven 
without  any  bleaching  effect;  and  further,  that  wax, 
which  the  sunlight  likewise  bleaches,  becomes  darker, 
rather  than  paler,  through  heat. 

As  we  remarked  before,  the  bleaching  effect  of  sun- 
light is  a  slow  process,  and  this  circumstance  renders 
the  phenomenon  less  striking.  A  sudden  and  rapid 
occurrence  surprises  us,  and  stirs  us  up  to  inquire  and 
to  reflect. 

In  the  mines  of  Friburg  is  now  and  then  found  a 
vitreous  dull-shining  silver  ore,  which,  on  account  of  its 
appearance,  is  called  horn  silver  (muriate  or  chloride  of 
silver). 

This  horn  silver  consists  of  silver  and  chlorine  in  chem- 
ical combination,  and  can  be  artificially  produced  by 
directing  chloric  gas  upon  metallic  silver.  This  horn  ore 
in  its  original  position  is  completely  colourless,  but  as 
soon  as  it  is  exposed  to  the  daylight  it  assumes,  in  a  few 
minutes,  a  violet  tint.  This  effect  of  light  has  long 
excited  the  astonishment  of  men  of  science. 

In  another  argentiferous  substance  this  phenomenon 
is  still  more  obvious.  If  nitric  acid  is  poured  upon 
silver,  it  dissolves  with  effervescence.  If  the  liquid  part 
of  the  solution  be  evaporated,  a  solid  mass  of '  crystals  is 
obtained,  which  is  not  silver  but  a  combination  of  silver 
with  nitric  acid.  This  nitric-acid  silver  is  totally  dif- 
ferent from  ordinary  silver ;  it  dissolves  easily  in  water, 
like  sugar ;  it  has  a  bitter,  disagreeable  taste ;  it  readily 
diminishes  and  destroys  organic  matter ;  and  it  is  there- 


DEVELOPMENT    OF    OUR    PHOTO-CHEMICAL   KNOWLEDGE.       5 


fore  used  as  a  corrosive  agent,  unfe  -tkajiame  of  lunar 
caustic,  or  nitrate  of  silver. 

It  has  been  long  known  that  the  fingers  which  grasp 
the  lunar  caustic,  or  anything  that  is  sprinkled  with  a 
solution  of  it,  quickly  assume  a  dark  colour.  This  can 
be  at  once  tested  by  moistening  a  small  piece  of  paper 
with  the  silver  solution,  allowing  it  to  dry,  and  then 
placing  it  in  the  light. 

These  properties  were  soon  made  use  of  to  produce 
a  so-called  indelible  ink,  which  ip/nothing  more  than  a 
solution  of  one  part  nitrate  of  silver  in  four  parts  of 
water,  combined  with  a  somewhat  thick  solution  of  gum. 
Written  characters  traced  with  it  upon  linen  cloth  are 
pale;  but,  when  dried  in  the  sunlight,  quickly  become 
dark  brown,  and  are  not  injured  by  washing.  Ink  of 
this  kind  is  much  used  in  hospitals  for  marking  linen. 
A  quill,  not  a  steel  pen,  must  be  made  use  of,  as  that 
metal  decomposes  the  nitrate  of  silver.  It  is  not 
unusual  to  print  the%  characters  by  means  of  wooden 
type. 

From  the  discovery  of  the  blackening  of  paper  saturated 
with  lunar  caustic  to  the  invention  of  photography  there 
was  but  a  step ;  yet  it  was  long  before  any  one  thought 
of  producing  pictures  by  the  help  of  light  alone,  and  still 
longer  before  these  attempts  were  crowned  with  success. 

Wedgwood,  the  son  of  the  celebrated  manufacturer 
of  porcelain  who  produced  the  still  popular  Wedgwood 
ware,  and  Davy,  the  celebrated  chemist,  made  the  first 
attempts  in  the  year  1802.  They  placed  flat  bodies,  such 
as  the  leaves  of  plants,  upon  lunar  caustic  paper.  Light 
was  thus  kept  from  the  superimposed  parts  of  the  paper, 
the  underlying  parts  remained  white,  whilst  the  un- 


6 


THE    CHEMISTRY    OF    LIGHT. 


covered  portions  of  the  paper  were  blackened  by  the 
light ;  and  thus  was  produced  a  white  outline,  or 
"  silhouette,"  of  the  superimposed  objects  upon  a  black 
ground.  (See  Figs.  1  and  2.) 


Fig.  2. 


Wedgwood  produced  in  this  manner  copies  of  draw- 
ings in  white  lines  upon  a  black  ground  made  upon 
glass,  and  this  process  became  the  basis,  in  modern 
times,  of  a  mode  of  treatment  which  attained  the 
highest  importance,  coming  under  the  name  of  the 
light-paws  process. 

Unfortunately  these  pictures  were  not  durable.  They 
had  to  be  kept  in  the  dark,  and  could  only  be  exhibited 
in  a  subdued -light.  If  they  remained  long  exposed  to 
the  light,  the  white  parts  also  became  black ;  and  thus 
the  picture  disappeared.  No  means  were  then  known  to 
make  the  pictures  durable,  that  is  to  say,  to  make  them 
light-resisting,  or  as  we  now  say,  to  fix  them.  But  the 
first  step  towards  the  discovery  of  photography  was 
made;  and  the  idea  of  producing  pictures  of  the  world 


DEVELOPMENT    OF    OUR   PHOTO- CHEMICAL   KNOWLEDGE.       7 

of  matter  without  the  help  of  the  draughtsman  became, 
after  these  first  attempts,  so  extremely  attractive  that, 
from  that  time,  both  in  England  and  in  France,  a  large 
number  of  persons  occupied  themselves  with  the  subject 
in  private  with  the  greatest  enthusiasm. 

It  is  clear  that  by  the  process  of  Wedgwood  and  Davy 
only  flat  bodies  could  be  copied,  and,  notwithstanding 
all  the  improvements  of  which  the  process  was  still 
susceptible,  it  admitted  of  only  a  limited  application. 

But  even  Wedgwood  seized  tjtfe  thought  whether  it 
were  not  possible  by  the  help  of  light  to  produce  pictures 
of  any  bodies  whatsoever  on  sensitized  paper.  He  tried 
to  effect  this  by  the  aid  of  an  interesting  optical  instru- 
ment wiiich  had  the  property  of  projecting  flat-shaded 
images  of  solid  objects.  This  instrument  is  the  camera 
obscura. 


If  a  small  hole  be  made  in  the  window  shutter  of  a 
completely  darkened  room  on  a  sunny  day,  a  clear 
image  of  the  landscape  will  be  seen  on  the  opposite  wall 
of  the  room. 

Let  a  be  a  poplar,  o  the  hole,  and  w  the  back  wall  of 


8 


THE    CHEMISTRY    OF   LIGHT. 


the  room,  then  from  each  point  of  the  poplar  rays  of 
light  are  projected  towards  the  hole,  and  beyond  that  in 
a  straight  line  to  the  wall.  It  is  now  clear  that  to  the 
point  of  light  can  only  arrive  from  the  point  a  of  the 
poplar,  which  is  situated  on  the  extension  of  the  line  a ' 
and  o.  Therefore  the  point  in  question  of  the  wall  can 
only  reflect  light,  which  in  its  colour  and  position,  corres- 
ponds to  the  point  a.  The  same  remark  applies  to  the 
points /and  g,  and  the  result  accordingly  is  that  on  the 
wall  an  inverted  image  of  the  tree  is  visible.  This  was 
first  observed  by  Porta  the  celebrated  Italian  natural 


philosopher  ol  the  16th  century,  whose  house,  we  are  told 
by  contemporaries,  was  seldom  free  from  visitors  in 
search  of  knowledge.  This  instrument  was  soon  im- 
proved by  substituting  a  small  box  (Fig.  4)  instead  of 
the  chamber,  which  box  in  place  of  a  solid  wall  had  a 
movable  unpolished  slide.  On  this  slide  the  image  of  an 
object  in  front  of  the  box  is  clearly  visible,  if  a  minute 
hole  is  made  in  the  front  partition,  which  answers  best  if 
composed  of  a  thin  tin  metal  plate.* 

These  images  appear  still  more  beautiful  if,  instead  of 

*  To  prevent  the  access  of  light  the  head  must  be  covered  with  a 
cloth. 


DEVELOPMENT    OF    OUR    PHOTO- CHEMICAL   KNOWLEDGE.       9 

a  hole,  a  glass  lens,  or,  as  it  is  called,  a  focal  lens,  is 
substituted.  This  focal  lens,  or  "  burning-glass,"  at  a 
certain  distance,  which  is  equal  to  that  of  its  focus, 
projects  a  distinct  image  of  the  objects — which  is  much 
better  defined  and  clearer  than  that  which  is  produced 
by  the  hole. 

In  this  improved  form  was  the  instrument  now  em- 
ployed by  Wedgwood  and  Davy.  Their  idea  was  to 
catch  a  small  picture  upon  the  unpolished  slide  by  means 
of  sensitized  paper.  They  fastened  a  pfece  of  paper 
saturated  with  salts  of  silver  upon  the  place  of  the 
image,  and  left  it  there  for  several  hours — unfortunately 
without  result.  The  pictures  were  not  distinct  enough 
to  make  a  visible  impression  upon  the  sensitized  paper ; 
or  the  paper  was  not  sufficiently  sensitive.  It  now 
became  necessary  to  find  a  more  sensitized  preparation 
to  catch  and  to  retain  the  indistinct  image ;  and-  this 
was  achieved  by  a  Frenchman — Nicephore  Niepce. 
He  had  recourse  to  a  very  peculiar  substance,  the 
sensitiveness  of  which  to  light  was  before  unknown  to  any 
one — asphaltum,  or  the  bitumen  of  Judaea.  This  black 
mineral  pitch,  which  is  found  near  the  Black  Sea,  the 
Dead  Sea,  the  Caspian,  and  many  other  places,  is  soluble 
in  ethereal  oils — such  as  oil  of  turpentine,  oil  of  lavender, 
besides  petroleum,  ether,  and  others.  If  a  solution  of 
this  substance  is  poured  upon  a  metal  plate,  and  allowed 
to  cover  the  surface,  a  thin  fluid  coating  adheres  to  it, 
which  soon  dries  and  leaves  behind  a  light  brown  film 
of  asphaltum.  This  film  of  asphaltum  does  not  receive 
a  darker  hue  in  the  light,  but  it  loses  by  light  its  property 
of  solubility  in  ethereal  oils. 

If  such  a  plate,  therefore,  is  put  in  the  place  of  the 


10  THE    CHEMISTRY    OF   LIGHT. 

small  image  of  the  camera  obscura,  the  asphaltum  coat- 
ing will  remain  soluble  on  all  the  dark  places  (shadows) 
of  the  image,  whilst  on  the  light  spots  it  will  remain 
insoluble.  The  eye,  it  is  true,  does  not  perceive  these 
changes.  The  plate  appears  .the  same  after  as  before 
being  exposed  to  the  influence  of  light.  But,  if  oil  of 
lavender  is  poured  over  the  coating  of  asphaltum  it 
dissolves  all  the  spots  that  had  remained  unchanged, 
and  leaves  behind  all  those  that  had  been  changed  by 
light,  that  is,  had  been  rendered  insoluble.  Thus,  after 
several  hours  exposure  in  the  camera  obscura,  and  subse- 
quent treatment  with  ethereal  oils,  Niepce  succeeded,  in 
fact,  in  obtaining  a  picture.  This  picture  was  very  im- 
perfect it  is  true,  but  still  interesting  as  a  first  attempt 
to  fix  the  images  of  the  camera,  and  still  more  interest- 
ing as  evidence  that  there  are  bodies  which  lose  their 
solubility  in  the  sunlight.  These  facts  wrere  again 
verified  long  after  the  death  of  Niepce,  and  they  led  to 
one  of  the  finest  applications  of  photography,  that  of 
heliography,  or  the  combination  of  photography  with 
copper-plate  printing,  which  Niepce  himself  to  all  ap- 
pearance had  already  known. 

A  copper-plate  print  is  produced  in  this  way: — A 
smooth  copper  plate  is  engraved  with  the  burin  (or 
graving  tool),  that  is  to  say,  the  lines  which  should 
appear  black  in  the  picture  are  deeply  incised  in  the 
plate.  In  producing  impressions,  ink  is  first  rubbed  into 
these  incisions,  and  then  a  sheet  of  paper  is  placed  upon 
the  plate  and  subjected  to  the  action  of  a  cylindrical 
press ;  after  which  the  ink  passes  over  the  paper  and 
produces  the  copper-plate  impression.  Niepce  en- 
deavoured to  substitute,  by  the  help  of  light,  a  less 


DEVELOPMENT    OF   OUR   PHOTO-CHEMICAL   KNOWLEDGE.    11 

laborious  process  than  the  troublesome  one  of  cutting  by 
the  engraver.  To  effect  this  he  covered  the  copper  plate 
with  asphaltum  as  before  stated,  and  exposed  this  to  the 
light  beneath  a  drawing  on  paper.  In  this  case  the 
black  lines  of  the  drawing  kept  back  the  light;  and, 
accordingly,  in  these  places  the  asphaltum  coating  re- 
mained soluble ;  under  the  white  paper,  on  the  contrary, 
it  became  insoluble.  Therefore,  when  lavender  oil  was 
afterwards  poured  over  the  plate,  the  parts  of  the 
asphaltum  which  had  become  insoluble  adhered  to  the 
plate,  whilst  the  soluble  parts  were  dissolved  and  re- 
moved ;  and  thus  the  plate  in  those  places  was  laid  bare. 
Thus  a  film  of  asphaltum  is  obtained  on  the  plate  in 
which  the  original  drawing  appeared  as  if  engraved. 

If  a  corrosive  acid  is  now  poured  on  such  a  plate, 
it  can  only  act  on  the  metal  in  those  places  where 
it  is  not  protected  by  the  asphaltum;  and  in  these 
places  the  metal  plate  was  in  fact  eaten  into.  Thus 
an  incised  drawing  upon  a  metal  was  effected  through 
the  corrosive  operations  of  the  acid,  and  a  plate 
was  obtained  which,  when  rubbed  clean,  could  be  used 
for  impressions  like  an  engraved  copper  plate.  Copper- 
plate impressions  of  this  kind  have  been  found  amongst 
the  papers  left  behind  by  Niepce,  which  he  called 
"heliographs,"  and  showed  to  his  friends  as  far 
back'  as  1826.  This  method,  in  an  improved  form, 
is  still  in  use  at  the  present  day,  especially  in  the 
printing  of  paper  money,  when  it  is  requisite  to  produce 
a  number  of  engraved  plates  which  are  all  to  be 
absolutely  alike,  so  that  one  piece  of  paper  money  may 
perfectly  correspond  to  another,  and  may  therefore  be 
distinguished  from  counterfeits.  In  this  wray  the  arms 


12  THE    CHEMISTRY   OF   LIGHT. 

and  the  inscription  on  the  upper  side  of  the  Prussian 
ten  thaler  notes  are  printed  off  from  heliographic  plates. 
Thousands  of  people  carry  photographic  impressions 
in  their  pocket-books,  without  knowing  it.  Nor  is  there 
any  occasion  to  fear  that  these  notes  can  be  imitated 
easily  by  the  help  of  photography  or  heliography.  We 
shall  show  later  on,  that  the  ground  tint,  the  paper 
itself,  and  the  colour  of  the  inscription  present  well- 
devised  obstacles  to  all  such  imitations,  and  make  them 
very  difficult,  if  not  impossible. 

Niepce's  impressions  were  undoubtedly  very  imperfect, 
and  therefore  remained  unnoticed.  He  himself  gave 
them  up,  and  again  entered  upon  a  series  of  experiments 
to  fix  the  charming  images  of  the  camera  obscura.  In 
1829  Daguerre  joined  him;  and  both  carried  on  ex- 
periments in  common  until  1833,  when  Niepce  died 
without  having  obtained  the  reward  of  his  long-continued 
efforts. 

Daguerre  went  on  with  his  experiments ;  and  he 
would  not,  perhaps,  have  carried  them  any  further  if 
a  fortunate  accident  had  not  worked  in  his  favour. 

He  made  experiments  with  iodide  of  silver  plates. 
Then  he  produced,  by  exposing  silver  plates  to  the 
vapour  of  iodine,  a  peculiar  and  very  volatile  chemical 
element.  Under  this  treatment,  the  silver  plate  assumed 
a  pale  yellow  colour,  which  is  peculiar  to  the  com- 
bination of  iodine  and  silver.  These  plates  of  iodide 
of  silver  are  sensitive  to  light,  they  take  a  brown  colour 
when  exposed  to  it,  and  an  image  is  soon  produced  upon 
them  when  they  are  exposed  to  the  operations  of  light 
in  the  camera.  A  very  long  exposure  to  light,  however, 
is  necessary  to  this  end ;  and  the  thought  could  scarcely 


DEVELOPMENT   OF   OUR   PHOTO -CHEMICAL   KNOWLEDGE.    13 

have  arisen  of  taking  the  likeness  of  any  person  in 
this  manner,  for  he  would  have  been  obliged  to  remain 
motionless  for  hours  to  obtain  it. 

One  day  Daguerre  placed  aside  as  useless,  in  a  closet 
in  which  were  some  chemical  substances,  several  plates 
that  had  been  exposed  too  short  a  time  to  the  light,  and 
therefore  as  yet  showed  no  image.  After  some  time 
he  looked  by  accident  at  the  plates,  and  was  not  a  little 
astonished  to  see  an  image  upon  them.  He  immediately 
divined  that  this  must  have  arisen  through  the  operation 
on  the  plates  of  some  chemical  substance  which  was 
lying  in  the  closet.  He,  therefore,  proceeded  to  take 
one  chemical  out  of  the  closet  after  the  other,  and 
placed  in  it  plates  recently  exposed  to  the  light,  when, 
after  remaining  there  some  hours,  images  were  again 
produced  upon  them.  At  length  he  had  removed  in 
succession  all  the  chemical  substances  from  the  closet ; 
and  still  images  were  produced  upon  the  plates  that  had 
been  exposed  to  the  light.  He  was  now  on  the  point 
of  believing  the  closet  to  be  bewitched,  when  he  dis- 
covered on  the  floor  a  shell  containing  quicksilver, 
which  he  had  hitherto  overlooked.  He  conceived  the 
notion  that  the  vapour  from  this  substance — for  mer- 
cury gives  off  vapour  even  at  an  ordinary  temperature — 
must  have  been  the  magic  power  which  produced  the 
image.  To  test  the  accuracy  of  this  supposition,  he 
again  took  a  plate  that  had  been  exposed  to  light  for  a 
short  time  in  the  camera-obscura,  and  on  which  no 
image  was  yet  visible.  He  exposed  this  'plate  to  the 
vapour  of  quicksilver,  and,  to  his  intense  delight,  an 
image  appeared,  and  the  world  was  again  enriched  by 
one  of  its  most  beautiful  discoveries. 

2 


14  THE   CHEMISTRY   OF   LIGHT. 


CHAPTEE  II. 

THE  DAGUERREOTYPE. 

Its  Publication  and  Spread — Its  Path  of  Development — Improvements- 
Discovery  of  the  Portrait  Lens —./Esthetic  effects  of  the  Daguerreo- 
type. 

MANY  persons  at  the  present  day  who  have  before  their 
eyes  the  grand  productions  of  paper  photography,  such, 
for  example,  as  portraits  of  life-size,  view  doubtless  with 
pity,  or  even  contempt,  the  little  pictures  that  were  called 
daguerreotypes  from  their  inventor.  The  appearance  of 
these  pictures  was  no  doubt  injured  by  the  ugly  mirror- 
like  dazzle  which  prevented  a  clear  view  of  them.  It 
was  different  in  the  year  1830,  when  Daguerre's  discovery 
was  first  spread  abroad  by  report.  Pictures  were  said  to 
be  produced  without  a  draughtsman  by  the  operations 
of  the  sun's  rays  alone.  That  was  of  itself  wonderful; 
but  it  was  still  more  wonderful  that,  by  the  mysterious 
operation  of  light,  every  body  impressed  its  own  image 
on  the  plate.  How  many  extravagant  hopes  and  how 
many  evil  prognostications  were  associated  with  the 
report  of  this  mysterious  invention. 

It  was  prophesied  that  painting  would  come  to  an  end, 
and  that   artists  would  die  of  starvation.     Every  one 


THE    DAGUERREOTYPE.  15 

hoped  that  he  could  with  ease  obtain  images  of  any 
objects  which  he  desired. 

A  friend  is  leaving  home :  in  an  instant  his  image  is 
permanently  retained  at  the  moment  of  departure.  A 
joyous  company  is  assembled :  a  picture  is  taken  of  it  at 
once  as  a  souvenir.  All  objects  were  thus  retained  as 
pictures  by  the  chemical  action  of  the  rays  of  light :  the 
landscape  glowing  with  the  magical  effects  of  sunset, 
the  favourite  spot  in  the  garden,  the  daily  motley  move- 
ment of  the  streets,  of  men,  of  animals,  of  everything 
alive. 

Then  came  sceptics  who  declared  the  whole  thing 
impossible.  These  persons  were  reduced  to  silence  by 
the  testimony  of  Humboldt,  Biot,  and  Arago,  the  three 
celebrated  natural  philosophers  to  whom  Daguerre  dis- 
closed his  secret  in  1838.  The  excitement  grew. 
Through  the  influence  of  Arago  an  application  was 
made  to  secure  to  Daguerre  a  yearly  pension  of  6000 
francs,  provided  he  made  public  his  discovery.  The 
French  Chamber  of  Deputies  agreed ;  and,  after  a  long 
and  tiresome  delay,  the  discovery  was  at  length  disclosed 
to  the  expectant  world. 

It  was  at  a  memorable  public  seance  of  the  French 
Academy  of  Sciences  in  the  Palais  Mazarin,  on  the  19th 
of  August,  1830,  that  Daguerre,  in  the  presence  of  all 
the  great  authorities  in  art,  science,  and  diplomacy, 
who  were  then  in  Paris,  illustrated  his  processes  by 
experiment. 

Arago  declared  that  "France  had  adopted  this  dis- 
covery, and  was  proud  to  hand  it  as  a  present  to 
the  whole  world ;  "  and  henceforth,  unhindered  by  the 
quackery  of  mystery,  and  unlimited  by  the  right  of 


16  THE    CHEMISTRY   OF   LIGHT. 

patent,*  the  discovery  of  Daguerre  made  the  round  of 
the  civilized  world. 

Daguerre  quickly  gathered  round  him  a  number  of 
disciples  from  all  quarters  of  the  globe ;  and  they  trans- 
planted the  process  to  their  homes,  and  became  in  their 
turn  centres  of  activity,  which  daily  added  to  the 
number  of  disciples  of  the  art. 

Sachse,  a  dealer  in  art  still  living  in  Berlin,  was 
initiated  into  Daguerre's  discovery  on  the  2nd  of  April, 
1839,  and  was  appointed  Daguerre's  agent  in  Germany 
on  the  22nd  of  September,  four  weeks  after  the  publica- 
tion of  the  discovery.  Sachse  had  already  produced  the 
first  picture  at  Berlin.  These  pictures  were  gazed  at  as 
wonders,  and  each  copy  was  paid  for  at  the  rate  of  from 
£1  to  £2 ;  while  original  impressions  of  Daguerre  fetched 
as  much  as  £4  16s.  Sd.  (120  francs).  On  the  30th  of 
September  Sachse  made  experiments  in  the  Park  of 
Charlottenburg,  in  the  presence  of  King  Frederick 
William  the  Fourth.  In  October  the  earliest  Daguerre 
apparatuses  were  sold  in  Berlin.  The  first  set  of 
apparatus  was  purchased  by  Beuth  for  the  Eoyal 
Academy  of  Industry  at  Berlin ;  and  it  is  still  to  be  seen 
there.  After  the  introduction  of  the  apparatus,  it  was 
in  the  power  of  every  one  to  carry  out  the  system ;  and 
a  great  number  of  daguerreotypists  started  into  exist- 
ence. Men  of  science,  too,  cultivated  (more  than  they 
do  now)  the  new  art :  among  others,  the  natural 
philosophers,  Professors  Karsten,  Moser,  Norrenberg, 
Von  Ettinghausen — nay,  even  ladies,  as  Frau  Professor 
Mitscherlich.  The  first  objects  photographed  by  Sachse 

*  It  was  only  in  England  that  the  process  was  patented,  before  its 
publication  on  the  15th  of  July,  1839. 


THE   DAGUERREOTYPE.  17 

were  architectural  views,  statuary,  and  paintings,  which 
for  two  years  found  a  ready  sale  as  curiosities.  It  was 
in  1840  that  he  first  represented  groups  of  living  persons, 
and  in  this  way  photography  became  especially  an  art 
of  portraiture.  It  made  the  taking  of  portraits  its  prin- 
ciple means  of  support,  and  in  two  years  there  were 
daguerreotypists  in  all  the  capitals  of  Europe. 

In  America  a  painter,  Professor  Morse,  afterwards 
the  inventor  of  the  Morse  telegraph,  was  the  first  to* 
prepare  daguerreotypes;  and  his  coadjutor  was  Professor 
Draper. 

Let  us  now  consider  more  closely  the  process  em- 
ployed in  producing^daguerreotype  plates.  A  silver 
plate,  as  I  have  said,  or  in  the  place  of  it  a  silver-plated 
copper  plate,  serves  as  a  plane  surface  for  the  image. 
This  is  rubbed  smooth  by  means  of  tripoli  and  olive 
oil ;  and  then  it  receives  its  highest  polish  with  rouge 
and  water  and  cotton.  It  is  only  a  plate  so  extremely 
well  polished  that  can  be  used  for  the  process.  This 
burnished  plate  is  placed  with  its  polished  side  upon  an 
open  square  box,  the  floor  of  which  is  strewn  with  a 
thin  layer  of  iodine.  This  iodine  evaporates,  its  vapours 
come  into  contact  with  the  silver,  and  instantly  combine 
with  it.  By  this  means  the  plate  first  assumes  a  yellow 
straw-colour,  next  red,  then  violet,  and  lastly  blue.  The 
plate  is  then  protected  from  the  light ;  next  it  is  placed 
in  the  camera  obscura,  where  the  image  on  the  ground- 
glass  slide  is  visible,  and  "  exposed  "  for  a  certain  time. 
It  is  afterwards  brought  back  into  the  tlark,  and  put 
into  a  second  box,  upon  the  metal  floor  of  which  there 
is  quicksilver.  This  quicksilver  is  slightly  warmed  by 
means  of  a  spirit  lamp.  At  first  not  a  trace  of  the 


18  THE    CHEMISTKY   OF   LIGHT. 

image  is  visible  on  the  plate.  This  first  conies  out  when 
the  vapour  of  the  quicksilver  precipitates  itself  upon 
the  places  affected  by  the  light,  and  the  result  is  in  pro- 
portion to  the  effectiveness  of  the  operation  of  light. 
During  this  process  the  quicksilver  condenses  itself  into 
very  minute  white  globules,  which  can  be  very  well 
discerned  under  the  microscope.  This  operation  is 
called  the  development  of  the  picture. 

After  the  development  the  iodide  of  silver,  being 
sensitive  to  light,  must  be  removed  to  render  the  image 
durable,  that  is,  "to  fix  it."  This  is  effected  by  using 
a  solution  of  hypo-sulphite  of  soda,  which  dissolves 
the  iodide  of  silver.  Nothing  more  is  required  after  this 
than  to  wash  with  water  and  dry,  and  the  daguerreotype 
is  completed.  Sometimes,  in  order  to  protect  the  picture, 
it  was  usual  to  gild  it.  A  solution  of  chloride  of  gold 
was  poured  over,  and  then  it  was  warmed ;  a  thin  film 
of  gold  was  deposited,  which  contributed  essentially  to 
the  durability  of  the  pictures.  A  picture  of  this  nature, 
however,  remains  always  exposed  to  injury,  and  requires 
the  protection  of  frame  and  glass. 

Daguerre's  first  pictures  needed  an  exposure  of  20 
minutes  to  the  light — too  long  for  taking  portraits.  But 
soon  after  it  was  found  that  bromine,  a  rare  substance 
having  many  points  of  resemblance  with  iodine,  employed 
in  combination  with  the  latter,  produces  much  more 
sensitive  plates,  which  required  far  less  time,  perhaps 
not  more  than  from  one  to  two  minutes,  for  exposure. 

Many  of  us,  perhaps,  still  remember  the  early  period 
of  photography,  when  persons  were  obliged  to  sit  in  the 
full  sunlight,  and  allow  the  dazzling  rays  to  fall  directly 
upon  the  face — a  torture  which  is  clearly  marked  and 


THE    DAGUERREOTYPE.  19 

visible  on  the  portraits  still  preserved  of  these  photo- 
graphic victims,  in  the  blackened  shadows,  the  distorted 
pauscles,  and  the  half-closed  eyes.  These  caricatures 
could  certainly  not  bear  any  comparison  with  a  good 
portrait  from  the  life,  nor  probably  would  portrait-photo- 
graphy have  ever  had  such  success  if  it  had  not  suc- 
ceeded in  obtaining  the  exposure  to  a  moderated  light. 
This  was  obtained  by  the  invention  of  a  new  lens — the 
double  objective  portrait  lens  of  Professor  Petzral,  of 
Vienna. 

This  new  lens  was  distinguished  by  the  fact  that  it 
produced  a  much  clearer  pictupe^than  the  old  lens  of 
Daguerre,  because  it  was  now  possible  to  take  impres- 
sions from  less  dazzlingly  lighted  objects.  This  lens  was 
invented  by  Petzral  in  1841.  Voigtlander  ground  the 
lens  according  to  his  directions,  and  soon  one  of  Voigt- 
lander's  lenses  became  indispensable  to  every  daguerreo- 
typist.  By  employing  iodide  of  bromium  and  Voigtlander 's 
lens,  the  process  of  exposure  was  made  a  matter  of 
seconds. 

The  daguerreotype  art  had  thus  reached  its  zenith. 
However  delicate  pictures  produced  appeared,  it  was 
found,  after  the  first  enthusiasm  had  gone,  and  had 
given  place  to  a  cold  spirit  of  criticism,  that  much  still 
remained  to  be  desired. 

First,  the  gloss  and  brilliancy  of  the  pictures  make 
it  difficult  to  look  at  them.  Then  there  are  several 
marked  deviations  from  nature:  yellow  objects  often 
produced  little  or  no  effect,  or  gave  a  black  impression; 
on  the  other  hand,  blue  objects,  which  appear  dark  to 
the  eye,  frequently,  though  not  always,  came  out  white. 

This  is  still  the  case  in  photography,  only  now  the 


20  THE    CHEMISTRY   OF   LIGHT. 

attempt  is  made  to  diminish  this  defect  by  subsequent 
treatment  of  the  plate  (negative  re-touching). 

But  still  a  well-grounded  aesthetic  objection  was 
brought  against  these  pictures. 

It  was  indisputable  that  the  daguerreotype  greatly 
surpassed  painting  by  the  wonderful  clearness  of  detail, 
by  the  fabulous  truthfulness  with  which  it  reproduced 
the  outlines  of  objects.  The  daguerreotype  plate  gives 
more  than  the  artist,  but  for  that  very  reason  it  gives 
too  much.  It  reproduces  the  subordinate  objects  as 
faithfully  as  the  principal  object  in  the  picture. 

Let  us  take  the  simplest  case — a  portrait.  A  painter, 
when  he  paints  a  portrait,  does  not  by  any  means  paint 
all  that  he  sees  in  nature.  The  original  wears,  perhaps, 
a  shabby  coat,  which  shows  a  good  many  creases,  per- 
haps a  spot  of  grease,  or  a  patch  ;  but  this  does  not 
distress  the  painter  in  the  least,  for  he  leaves  out  these 
accidental  details.  In  the  same  spirit,  if  the  original  is 
seated  before  a  whitewashed  wall,  the  artist  by  no 
means  puts  a  whitewashed  wall  into  his  picture,  for  he 
can  leave  out  all  that  is  displeasing,  or  add,  on  the 
contrary,  what  he  wishes. 

It  is  different  in  photography.  This  art,  in  taking 
portraits,  reproduces  all  those  minor  accessories  which 
disturb  the  picture,  as  faithfully  as  the  principal  object  in 
it — the  individual  himself.  Another  point  must  be  added 
to  this.  The  different  elements  admitted  by  the  painter 
into  his  picture  are  by  no  means  made  equally  pro- 
minent. The  head  is  the  chief  consideration  in  every 
portrait.  The  painter  accordingly  gives  his  best  skill 
and  care  to  the  painting  of  the  head  in  the  most  careful 
manner.  At  the  very  least  he  puts  the  head  in  the 


THE    DAGUERREOTYPE.  21 

strongest  light,  and  leaves  the  rest  of  the  picture  in  a 
half-shade.  But  in  photography  it  is  by  no  means  the 
head  which  is  generally  the  most  prominent — frequently 
it  is  a  chair,  or  part  of  a  background ;  and  this  detracts 
considerably  from  the  effect  of  the  picture.  Finally,  the 
expression  of  the  face  is  an  important  point  in  a  picture; 
and  this  varies  with  the  mood  of  the  sitter.  Photo- 
graphy gives  the  expression  which  the  original  had  at 
the  moment  the  picture  was  taken.  Now  the  expression 
varies,  and  is  affected  by  a  slight  annoyance,  a  vexatious 
circumstance,  ennui,  or  even  by  the  motionless  attitude 
which  has  to  be  observed  during  the  process ;  and  hence 
the  portrait  often  looks  strange  and  unnatural. 

It  is  quite  otherwise  with  painting.  The  painter  has 
longer  sittings  of  the  original  than  the  photographer; 
he  soon  learns  to  distinguish  the  accidental  frame  of 
mind  from  the  characteristic  expression  of  the  face, 
and  thus  he  is  in  a  condition  to  produce  a  portrait  much 
more  closely  corresponding  with  the  character  of  the  ori- 
ginal than  that  of  the  photographer  can  ever  be.  This 
naturally  applies  only  to  paintings  of  masters  of  the  first 
order.  In  the  portraits  of  the  dauber,  none  of  these 
advantages  are  found  ;  and  this  large  class  disappeared, 
like  bats  before  the  light,  when  the  art  of  sun-painting 
suddenly  rose  upon  the  world.  Many  of  these  themselves 
adopted  the  new  art  and  attaine'd  to  greater  results  than 
they  could  have  done  as  painters. 

The  artist  of  merit  has  no  cause  to  fear  photography. 
On  the  contrary,  it  proves  advantageous  -to  him  by  the 
fabulous  fidelity  of  its  drawing — through  it  he  learns  to 
reproduce  the  outline  of  things  correctly — nor  can  it  be 
disputed  that,  since  the  invention  of  photography,  a 


22  THE    CHEMISTRY    OF    LIGHT. 

decidedly  greater  study  of  nature  and  a  greater  truth- 
fulness are  visible  in  the  works  of  our  ablest  painters. 

We  shall  see  further  on,  how  even  photography  appro- 
priated the  aesthetic  principles  according  to  which 
painters  proceed  in  preparing  their  portraits,  and  how 
thereby  a  certain  artistic  stamp  was  given  to  these  pro- 
ductions, which  raised  them  far  above  those  of  the  early 
period.  But  this  result  was  only  possible  when  the 
technical  part  of  photography  had  been  brought  to  per- 
fection, and  a  material  better  adapted  to  artistic  work 
than  an  unyielding  silver  plate  had  been  introduced. 


CHAPTEE  III. 

PAPER  PHOTOGRAPHY  AND  THE  LICHT-PAUS,  OR  NEW 
TALBOT  PROCESS. 

Talbot's     Paper    Photographs — Licht-Paus    Paper— Leaf -prints — LicJit- 
Paus  Process  and  its  Application. 

IN  the  same  year  that  Daguerre  published  his  process 
for  the  production  of  images  on  silver  plates,  Fox  Talbot 
gave  to  the  world  a  process  for  preparing  drawings  on 
paper  by  the  help  of  light.  Talbot  was  an  English 
gentleman  of  fortune,  who,  like  many  Englishmen  of 
leisure  and  means,  employed  his  time  in  scientific  obser- 
vations. He  plunged  paper  into  a  solution  of  kitchen 
salt,  dried  it,  and  then  put  it  into  a  solution  of  silver. 
In  this  way  he  obtained  a  paper  which  was  much  more 
sensitive  to  light  than  that  employed  by  Wedgwood. 
He  employed  this  paper  in  copying  the  leaves  of  plants. 
Talbot  himself  says,  "  Nothing  gives  more  beautiful 
copies  of  leaves,  flowers,  etc.,  than  this  paper,  especially 
under  the  summer  sun;  the  light  works  through  the 
leaves,  and  copies  even  the  minutest  veins." 

This  is  no  exaggeration  of  Talbot.  In  the  hands  of 
the  author  there  are  impressions  of  tjiis  kind,  from 
Talbot's  own  hand,  which  show  excellently  well  the 
venous  structure. 

The  pictures  copied  in  this  way  in  the  sunlight  are 


24 


THE    CHEMISTRY   OF   LIGHT. 


naturally  not  durable,  because  the  paper,  by  having 
salts  of  silver  in  its  composition,  is  still  sensitive  to 
light.  But  Talbot  offered  the  means  of  fixing  the 
pictures — he  plunged  them  in  a  hot  solution  of  kitchen 
salt ;  in  this  way  the  greater  part  of  the  salts  of  silver 
was  removed,  and  the  pictures  did  not  become  obscure 
to  any  considerable  extent  in  the  light. 


Fig.  5. 


The  celebrated  Sir  John  Herschel  carried  out  this 
fixing  process  even  more  successfully  by  plunging  the 
pictures  into  a  solution  of  hypo-sulphite  of  soda.  This 
salt,  which  dissolves  all  the  salts  of  silver,  was  at  that 
time  very  expensive,  costing  6  shillings  per  pound.  The 
production  of  this  salt  soon  kept  pace  with  the  increasing 


PKOCESS.    25 

demands  of  photography,  and  now  it  is  offered  for  sale  by 
the  ton,  and  at  as  low  a  rate  as  6%d.  the  pound. 

By  this  means  the  production  of  a  durable  sun-picture 
on  paper,  which  Wedgwood  had  in  vain  attempted,  was 
rendered  possible.  This  method  gave,  no  doubt,  only 
pictures  of  flat  objects  which  could  be  easily  pressed  on 
paper ;  for  instance,  leaves  of  plants,  patterns  of  stuffs. 
The  process  has  lately  been  resumed,  after  it  had  almost 
been  forgotten.  Charming  ornaments  of  leaves,  different 
plants,  and  flowers  were  produced ;  and  these  copies  were 
proportionally  more  beautiful  than  the  earlier  ones, 
because  a  much  finer  and  even-surfaced  paper  than  that 
of  Mr.  Talbot  has  recently  passed  into  trade,  under  the 
name  of  licht-paus  papier.*  These  prints  are  much  liked 
in  America.  We  give  on  the  accompanying  page  a 
faithful  imitation  of  one  of  these  leaf-prints. 

Since,  by  the  cheapness  of  sensitive  paper,  the  pro- 
duction of  these  leaf-prints  has  been  made  very  easy, 
we  give  here  the  mode  of  producing  them  for  our  fair 
readers,  who  will  be  able  in  this  manner,  like  their 
sisters  in  America,  to  make  ornamental  pictures  for  the 
adornment  of  lamp  shades,  portfolios,  and  similar  things. 

The  leaves — especially  ferns  and  the  like — are  suitably 
chosen,  pressed  between  blotting-paper  and  dried,  then 
gummed  on  one  side  and  grouped  gracefully  by  the  fair 
artist  upon  a  glass  slab  or  plate,  in  a  small  frame 
(Fig.  6).  As  soon  as  the  whole  is  dry,  the  impression 
can  be  at  once  commenced,  t 

*  This  paper  is  produced  by  Mr.  Remain  Talbofc,  11,  &arlstrasse,  Berlin. 

f  These  wooden  frames,  called  copying  frames,  dishes,  and  fixing  salt,  are 
also  manufactured  by  Mr.  Talbot,  at  Berlin.  There  is  now  even  a  small 
plaything  of  this  kind  on  sale,  known  by  the  name  of  the  "  sun-copying 
machine." 


26  THE    CHEMISTRY   OF    LIGHT. 

A  small  piece  of  sensitized  paper  is  placed  on  the 
leaves  after  they  are  arranged,  the  two  wooden  lids,  h  h, 
are  laid  upon  it,  and  fastened  down  by  means  of 
two  .little  cross-bar  pieces  of  wood,  x  x,  and  then  the 
whole  is  exposed  to  the  light,  the  glass  side  upper- 
most. The  sheet  of  paper  very  soon  assumes  a  brown 
colour,  where  it  is  not  covered  by  the  leaves,  and 

A 


Fig.  6. 

ultimately  it  receives  a  decided  bronze  tint.  The  light 
also  penetrates  partially  through  the  leaves,  and  colours 
the  paper  lying  under  them  brown.  It  is  easy  to  discern 
how  far  the  colouring  has  passed  under  the  leaves,  if 
one  of  the  cross-bars,  x,  and  the  half  cover,  h,  are 
removed,  and  the  paper  is  lifted  up. 
£ 


As  soon  as  the  impression  is 'dark  enough — it  is  quite 
a  matter  of  taste  whether  the  shade  be  dark  or  light— 
the  paper  is  taken  out  and  placed  for  a  time  in  a  dark 


PAPER  PHOTOGRAPHY  AND  THE  "  LIGHT -PAUS  "  PROCESS.  27 

closet.  Several  pictures  can,  in  like  manner,  be  taken  one 
after  the  other,  and  these  can  be  afterwards  fixed,  that 
is,  made  permanent  in  the  light. 

To  this  end  the  picture  is  placed  in  a  flat  dish  (Fig.  8) 
containing  water,  for  about  five  minutes,  and  then  in 
a  second  dish  in  which  a  solution  of  twenty  grammes 
of  fixing  natrium  have  been  combined  with  a  hundred 
grammes  of  wrater.  The  moment  the  impression  is 
dipped  in  this  it  becomes  of  a  yellowish  brown.  After 
the  impression  has  remained  ten  minutes  in  the  fixing 
solution — several  leaves  can  be  ftnmersed  in  succession 
— it  is  taken  out  and  placed  in  fresh  water  (most  con- 
veniently in  a  saucer).  This  operation  of  placing  in 
fresh  water  is  repeated  from  four  to  six  times,  the  picture 
being  left  in  the  water  three  minutes  each  time0 


Fig.  8, 

Afterwards  the  pictures  are  placed  on  blotting  paper 
and  suffered  to  dry ;  they  can  then  be  pasted  upon  card- 
board, thick  paper,  linen,  glass,  or  wood. 

To  many  persons  this  process  will  appear  an  agree- 
able pastime,  but  latterly  it  has  gained  increasing  con- 
sideration as  an  aid  to  the  copying  of  drawings,  maps, 
plans,  copper-plate  impressions,  and  so  forth. 

This  work  of  copying;  which  used  to  cost  the  artisan 
and  artist  many  hours  of  time  and  labour,  and  yet  was 
inaccurate,  can  be  accomplished  with  the  least  possible 
trouble  by  the  help  of  the  process  described  above. 


28  THE    CHEMISTRY   OF   LIGHT. 

Let  the  reader  imagine  a  drawing  placed  on  a  piece  of 
sensitized  paper,  and,  after  being  firmly  pressed  together 
by  a  glass  slab,  exposed  to  the  light.  The  light  penetrates 
through  all  the  white  places  of  the  paper,  and  colours 
brown  those  parts  of  the  paper  lying  under  them ;  but 
the  black  lines  of  the  drawing  keep  back  the  light,  and 
thus  the  underlying  paper  becomes  white  in  these  places. 
Therefore,  if  sufficient  time  is  given  for  the  operation  of 
the  light,  a  white  copy  on  a  dark  brown  ground  is  ob- 
tained in  this  manner,  which  is  fixed  and  washed 
exactly  like  the  leaf-prints  described  above.  This  copy 
is  reversed  with  reference  to  the  original,  like  an  object 
and  its  reflection  in  a  mirror ;  in  other  respects  it  is  a 
faithful  representation,  stroke  for  stroke. 

We  give  in  Plate  II.  the  copy  of  a  woodcut  struck 
off  according  to  this  method.  This  copy  is  rather  too 
small,  but  the  largest  as  well  as  the  smallest  drawing 
can  be  copied  equally  well ;  and  copies  of  this  kind, 
from  drawings  of  the  size  of  4*11  feet,  are  made  in 
technical  offices,  in  mines,  and  in  the  manufactories  of 
machinery. 

Large  copper  frames  are  used  for  this  purpose,  re- 
sembling in  their  construction  the  small  frames  named 
above ;  and  large  wooden  dishes,  covered  with  a  coating 
of  asphaltum,  are  employed  for  fixing  and  moistening. 
This  operation  is  called  in  practice  licht-paus  process. 
The  black  copy  produced  by  it  is  called  a  negative 
picture,  but  a  second  white  copy  can  be  prepared  from 
this  by  placing  the  negative  upon  sensitized  paper ;  then 
the  light  shines  through  all  the  clear  lines,  and  colours 
the  paper  lying  under  them  of  a  dark  hue,  whilst  it 
remains  white  under  the  dark  places  of  the  negative. 


PAPER  PHOTOGRAPHY  AND  THE  "  LICHT-PAUS  "  PROCESS.    29 

In  this  manner  a  picture  is  produced  which  perfectly 
resembles  the  primary  original,  called  a  positive.  The 
washing  and  fixing  are  carried  out  just  as  scrupulously 
with  the  positive  as  with  the  negative.  Fig.  9  offers  a 
positive  of  this  kind  taken  from  the  negative,  Fig.  5. 

Thus  the  geographer  is  in  a  position  to  prepare  quickly 
faithful  copies  of  his  sketches  and  maps,  the  engineer 


Fig.  9. 

is  able  to  copy  the  drawings  of  machines  which  are  to 
serve  as  models  for  the  workmen,  and  the  student  can 
copy  illustrations  of  natural  history  which  *  are  to  assist 
him  in  his  studies.  In  the  process  of  copying,  the 
sensitized  paper — licht-paus  paper — must  closely  touch 
the  original  picture ;  therefore  the  former  must  be  placed 


30  THE    CHEMISTEY   OF   LIGHT. 

on  the  side  of  the  picture,  and  not  on  the  reverse  side  of 
the  original. 

This  process  has  already  done  good  service  in  military 
operations,  where  it  was  important  to  make  quickly  a 
copy  of  some  map  of  which  there  was  only  one  impres- 
sion. If  an  attempt  had  been  made  to  draw  a  copy  of 
the  map,  it  would  have  required  several  days  to  carry 
out,  nor  would  the  copy  have  been  as  correct  as  the 
licht-paus. 

It  is  remarkable  that  this  process,  so  important  for 
industry,  has  only  quite  recently  been  known  in  its  full 
value,  although  the  experiments  of  Talbot  have  been 
before  the  world  for  thirty  years.  The  explanation  of 
this  fact  is,  no  doubt,  to  be  found  in  the  circumstance 
that  the  paper  impressions  were  far  less  distinct  than 
now,  being  often  rendered  worthless  by  spots.  Another 
reason  has  been  that  the  preparation  of  the  paper 
requires  especial  care,  and  therefore  frequently  failed  in 
the  hands  of  the  inexperienced;  that  is,  of  those  who 
were  not  professional  photographers.  Further,  the 
papers  prepared  according  to  the  old  method  soon  spoil, 
and  had  on  that  account  to  be  used  immediately  after 
they  were  prepared. 

These  disadvantages  have  been  removed  by  the  inven- 
tion of  Eomain  Talbot's  licht-paus  paper,  which  is  sold 
ready  prepared,  and  can  be  kept  for  months  ;  and  by  this 
means  the  process  can  be  easily  made  available  by  every 
professional  man  and  amateur. 


CHAPTEE  IV. 

THE  DEVELOPMENT  OF  MODERN  PHOTOGRAPHY. 

Talbot's  Paper  Negatives — Photography  as  an  Art  for  Multiplying 
Copies — Services  of  Niepce  de  St.  Victor — White  of  Egg  Negatives 
— Gnn-Cotton  in  Photography — Collodion — Archer's  Negative  Pro- 
cess— White  of  Egg  Paper — Carte  de  Visite — Photographic  Album. 

THE  reader  has  already  learnt  in  the  previous  chapter 
what  constitutes  a  negative,  and  how  by  its  means  copies 
produced  by  light,  of  plane  objects,  can  be  obtained. 

Talbot,  the  inventor  of  this  paper  process,  carried  out 
further  researches,  in  order  to  represent  on  paper,  by 
the  help  of  the  camera-obscura,  material  objects  which 
cannot  be  pressed  together  with  sensitized  paper ;  for 
example,  a  person  or  a  landscape. 

He  attained  this  object  two  years  after  Daguerre's 
discovery,  by  means  of  paper  prepared  with  iodide  of 
silver. 

He  saturated  paper  in  a  solution  of  nitrate  of  silver, 
and  then  in  a  solution  of  iodide  of  potassium.  He  thus 
obtained  a  slightly  sensitized  paper,  but  one  that  could 
always  be  rendered  very  sensitive,  by  plunging  it  into 
pyrogallic  acid  and  silver  (gallate  of  silver).* 

*  The  nature  of  this  peculiar  process  is  explained  further  on. 


32  THE    CHEMISTRY   OF   LIGHT. 

When  .this  paper  was  exposed  to  the  light  in  the 
camera  obscura,  it  did  not  give  at  once  a  picture — this  was 
only  clearly  defined  after  lying  some  time  in  the  dark, 
or  by  subsequent  treatment  with  pyrogallic  acid  and  silver 
—but  it  came  out  as  a  negative,  and  not  as  a  positive. 
Thus  in  taking,  for  example,  a  portrait,  the  shirt  ap- 
peared black,  also  the  face;  while  the  coat,  on  the 
contrary,  came  out  white. 

The  picture  was  made  light-resisting  by  plunging  it  in 
a  solution  of  hypo-sulphite  of  soda. 

A  negative  thus  obtained  is  a  picture  on  a  plane 
surface  of  a  solid  object,  and  Talbot  prepared  positive 
pictures  from  negatives  of  this  kind. 

He  placed  the  negative  upon  a  piece  of  sensitive  paper 
saturated  with  chloride  of  silver,  as  described  in  the  last 
chapter,  and  let  the  light  work  upon  it.  This  shone 
through  the  white  places  of  the  negative,  and  imparted 
a  dark  colour  to  those  parts  of  the  sensitive  paper  lying 
under  them,  while  the  dark  places  of  the  negative  pro- 
tected the  paper  lying  under  them  from  the  effects  of  the 
light.  Thus  he  obtained  a  positive  picture  from  a  nega- 
tive. He  was  now  able  to  repeat  the  process  as  often  as 
he  pleased,  and  therefore  was  in  a  position  to  copy,  by 
means  of  the  light,  many  positives  from  a  single  negative. 
Photography  was  thus  classed  among  the  arts  that 
repeat  copies,  and  this  circumstance  exercised  an  im- 
portant result  on  its  future  development. 

Daguerre's  method  only  gave  a  single  positive  at  a 
time ;  if  more  were  required,  the  person  had  to  sit 
several  times.  In  Talbot's  method  a  single  seance 
sufficed  to  produce  hundreds  of  pictures. 

It  must  be  admitted  that  the  earlier  pictures  of  the 


THE  DEVELOPMENT  OF  MODERN  PHOTOGRAPHY.    33 

Talbot  process  were  not  remarkably  engaging.  Every 
roughness  of  the  paper  and  each  small  speck  of  dirt 
were  imprinted  on  the  positive,  which  could  not  be 
compared  in  point  of  delicacy  with  the  fine  daguerreo- 
types ;  but  the  method  was  soon  improved. 

Niepce  de  St.  Victor,  nephew  of  Nicophore  Niepce, 
the  friend  of  Daguerre,  had  the  happy  idea  of  substi- 
tuting glass  for  paper.  He  covered  glass  plates  with  a 
solution  of  white  of  egg,  in  which  iodide  of  potassium 
was  dissolved. 

A  solution  of  this  kind  can  be  easily  produced  by 
beating  up  the  white  of  egg  to  the  consistency  of  snow, 
and  allowing  it  to  deposit.  The  glass  plates,  after  being 
dried  and  covered  with  a  coating  of  white  of  egg,  were 
afterwards  dipped  in  a  solution  of  silverc  Iodide  of  silver 
was  formed  in  this  manner — the  whole  coating  of  the 
white  of  egg  was  coloured  yellow,  and  became  very 
sensitive  to  light. 

Niepce  put  these  glass  plates  into  the  place  of  the 
picture  in  the  camera-obscura,  and  suffered  the  light  to 
work  upon  it. 

Its  impression  was  at  first  invisible,  but  afterwards 
became  clearly  perceptible  when  the  picture  was  im- 
mersed in  a  solution  of  pyrogallic  acid.  Thus  Niepce 
obtained  a  negative  on  glass  without  the  blemishes  which 
appeared  on  paper  negatives. 

He  repeated  this  negative  exactly  according  to  the 
same  process  that  had  been  employed  by  Fox  Talbot, 
and  he  obtained  from  the  fine  negative  a  correspondingly 
fine  positive,  which  was  much  better  calculated  to  bear 
a  comparison  with  the  productions  of  Daguerre. 

Niepce  invented  his  method  in  1847.    It  excited  much 


34  THE   CHEMISTRY   OF   LIGHT. 

attention,  but  had  a  shady  side :  the  treatment  with  white 
of  egg,  salts  of  silver,  and  pyrogallic  acid  was  a  dirty 
process.  Therefore  the  method  appeared  to  many,  who 
had  been  accustomed  to  the  daguerreotype,  dirty  and 
unpleasant,  and  deterred  persons  from  trying  it. 

On  the  other  hand,  the  advantages  of  the  new  process 
in  repeating  impressions  was  so  evident  that  it  could 
not  be  overlooked;  therefore,  even  those  who  had  an 
antipathy  to  soil  their  fingers  nevertheless  zealously 
devoted  themselves  to  the  work. 

The  easy  decomposition  of  white  of  egg  was,  however, 
a  great  disadvantage  in  the  new  process.  They  sought 
to  avoid  this  by  adopting  a  more  durable  substance. 

This  was  afforded  by  a  new  discovery,  gun-cotton,  made 
by  Schonbein  and  Bottcher  in  1847.  Schonbein  found 
that  ordinary  cotton  saturated  in  a  mixture  of  nitric  acid 
and  sulphuric  acid  assumes  explosive  properties  similar 
to  those  of  gunpowder.  It  was  conceived  that  this 
substance  was  an  important  substitute  for  gunpowder, 
but  it  was  soon  found  that  its  explosive  property  was 
very  unequal,  being  sometimes  too  strong  and  at  other 
times  too  weak.  On  the  other  hand,  another  very 
useful  property  was  observed  in  the  same  body — that  of 
being  dissolved  in  a  mixture  of  alcohol  and  ether.  This 
solution  leaves  behind  it  a  transparent  membrane  form- 
ing an  excellent  sticking-plaster  for  wounds.  Thus  the 
same  substance  that  was  destined  to  be  a  substitute  for 
gunpowder,  as  a  destructive  agent  for  producing  wounds, 
became  actually  a  remedy  for  the  latter.  This  solution 
of  gun-cotton  was  called  collodion. 

The  thought  occurred  to  different  photographic  ex- 
perimenters to  try  this  substance  instead  of  the  white  of 


THE  DEVELOPMENT  OF  MODERN  PHOTOGRAPHY.    85 

-  -tl_   ^^^ 

egg,  by  coating  glass  plates  writh  it ;  but  the  attempts  did 
not  at  first  lead  to  any  satisfactory  results.  At  length 
Archer  published  in  England  a  full  description  of  a 
collodion  negative  process  surpassing  in  the  beauty  of 
results,  in  simplicity  and  security,  Niepce's  white  of  egg 
process. 

Archer  coated  glass  plates  with  collodion,  In  which 
salts  of  iodide  had  been  dissolved ;  he  plunged  this  in  a 
solution  of  silver,  and  thus  obtained  a  membrane  of 
collodion  saturated  with  sensitive  iodide  of  silver,  which 
he  then  exposed  in  the  camera. 

The  invisible  impression  of  the  light  thus  produced 
became  visible  by  pouring  gallic  acid  over  the  plate, 
or  the  still  more  powerful  chemical  agent,  pyrogallic 
acid ;  or,  instead  of  this,  a  solution  of  green  vitriol. 

A  very  delicate,  clear  negative  was  directly  obtained 
by  this  process,  which  yielded  much  more  beautiful 
impressions  on  paper  than  the  original  negative  paper 
of  Talbot.  A  very  essential  improvement  was  sub- 
sequently made  in  the  preparation  of  negative  paper 
by  coating  it  with  white  of  egg,  according  to  the  process 
of  Niepce  de  St.  Victor.  By  this  means  it  received  a 
brilliant  surface,  and  when  exposed  to  the  light  it  took 
a  more  beautiful  and  warmer  tone,  which  gave  the 
pictures  a  brighter  appearance  than  those  produced 
upon  the  ordinary  paper. 

Thus  the  Talbot-type,  which  at  first  seemed  hardly 
worth  notice  compared  with  the  process  of  Daguerre, 
was  gradually  so  perfected  by  successive  improvements, 
that  it  ultimately  took  precedence  of  Daguerre's.  After 
1853,  paper  pictures  on  collodion  negatives  came  more 
and  more  into  vogue,  the  demands  for  daguerreotypes 


36  THE    CHEMISTRY   OF   LIGHT. 

fell  off  and  soon  vanished  altogether,  and  were  produced 
only  here  and  there  in  America. 

The  collodion  process  is  now  the  one  universally 
employed.  It  acquired  an  immense  impetus  through 
the  introduction  of  carfces  de  visite.  These  small  por- 
traits, which  are  intended  to  be  given  away,  and  there- 
fore had  to  be  produced  in  large  numbers,  were  invented 
by  Disderi,  the  court  photographer  of  the  Emperor 
Napoleon,  and  obtained'  so  great  a  success  that  they 
were  immediately  introduced  into  all  circles,  and  soon 
became  a  necessity  for  everybody.  The  moderate  price 
at  which  these  portraits  were  sold  made  them  attractive 
to  the  smallest  purses,  and  the  general  public  crowded 
to  the  ateliers,  the  number  of  which  increased  daily. 

The  old-fashioned  album,  the  favourite  souvenir  of 
young  people,  was  now  superseded  by  the  carte  de  visite, 
and  the  portraits  of  friends  were  substituted  for  their 
written  words.  A  photographic  album  is  now  found  in 
every  home;  and  in  Berlin  alone  there  are  at  present 
more  than  ten  photographic  album  manufactories,  to 
satisfy  the  demand,  from  whence  they  are  exported  to  all 
parts  of  the  world. 


CHAPTEE  V. 

THE  NEGATIVE  PROCESS. 

The  Dark  Chamber — Light  Inoperative  Chemically — Plate-cleaning — 
Application  of  Collodion— Sensitizing — The  Camera— The  Arrange- 
ment— The  Exposure  to  Light — The  Developing  Process — The  For- 
tifying  Process — The  Fixing  Process — The  Varnishing. 

IN  the  previous  chapters  we  have  dwelt  on  the  develop- 
ment of  this  art,  and  we  are  now  able  to  feel  at  home 
in  the  atelier  of  a  photographer.  His  whole  business 
depends  on  the  chemical  operation  of  light, — and  yet 
the  scene  of  his  principal  activity  is  not  the  illuminated 
atelier,  but  a  dungeon,  in  which  the  deepest  night  pre- 
vails, and  which  is  called  the  dark  chamber.  The 
sensitized  plate,  which  has  to  be  exposed  to  the  light, 
and  to  respond  to  its  most  delicate  operations,  must 
be  generated  in  darkness,  in  the  dark  chamber.  This 
space,  surrounded  by  bottles  and  boxes,  and  crammed 
with  instruments,  is  the  narrow  world  'of  the  photo- 
grapher, out  of  which  he  issues  only  for  a*  few  minutes 
into  the  light  of  his  atelier,  in  order  to  return  directly 
with  his  illuminated  plate,  and  to  subject  this  to  various 
other  chemical  operations. 

Many  persons  believe  that  the  opening  and  shutting 


38  THE    CHEMISTRY   OF   LIGHT. 

of  the  slide  —  the  cover  or  lid  of  the  apparatus 
falsely  called  the  machine — is  the  chief  work  of  the 
photographer.  Nay,  it  is  related  of  a  certain  queen, 
that  she  thinks  she  is  photographing,  when  she  has  all 
the  necessary  apparatus  brought  and  prepared,  and 
then,  when  all  is  ready  for  the  result,  opens  and  shuts 
the  lid  of  the  objective — a  work  that  a  child  of  five 
could  do  equally  well.  But  this  operation  is  only  a  link 
in  a  great  chain  of  twenty-eight  operations,  through 
which  each  plate  must  pass  to  produce  even  a  negative, 
while  at  least  eight  further  operations  are  required  to 
throw  off  a  positive  from  this  negative. 

Let  us  look  a  little  closer  at  these  operations.  The 
appearance  of  a  dark  chamber  is  by  no  means  inviting. 
Even  where  the  greatest  order  prevails,  drops  of  solution 
of  silver  are  diffused  about,  and  black  spots  produced 
here  and  there.  To  this  must  be  added  a  permanent 
odour  of  the  vapour  of  collodion,  and  an  unavoidable 
dampness  from  the  necessary  washing  of  the  plates, — 
and  all  this  is  seen  in  the  hazy  chiaro-oscuro  of  a  gas 
or  petroleum  lamp  provided  with  a  yellow  shade,  or 
of  a  small  window  fitted  with  a  similar  glass  shade. 

The  remark  must  here  be  made  at  the  outset,  that 
the  dark  chamber  of  the  photographer  is  not  really 
completely  dark.  The  light  of  day  only  must  be  ex- 
cluded from  certain  operations ;  but  the  yellow  light  of 
the  lamp  is  innocuous. 

From  this  we  learn  the  important  distinction  between 
light  chemically  operative,  and  light  chemically  inopera- 
tive. The  light  of  the  sun  and  of  the  blue  heavens,  the 
electric  light,  and  the  magnesium,  are  chemically  very 
operative,  gas  light  and  petroleum  light  very  slightly  so ; 


TEE   NEGATIVE   PROCESS.  39 

whilst  the  yellow  light  of  a  spirit  lamp,  whose  wick  has 
been  rubbed  with  kitchen  salt,  is  entirely  inoperative. 
The  operative  light  of  day,  furthermore,  can  be  rendered 
inoperative  if  allowed  to  pass  through  a  yellow  or,  better 
still,  a  reddish-yellow  glass  shade.  The  light,  therefore, 
that  falls  through  the  yellow  window  of  a  dark  room  is 
chemically  inoperative,  or  in  so  slight  a  degree  operative 
that  it  no  longer  causes  any  disturbing  effect.  It  is  re- 
markable that  the  yellow  light  which  affects  our  eyes 
so  powerfully,  should  influence  the  photographical  plate 
hardly  at  all.  Up  to  the  present  time  this  fact  has 
not  been  sufficiently  explained.  It  has  disadvantages 
for  practical  photography ;  for  example,  a  yellow 
garment  becomes  easily  black  in  photography,  a  yellow 
complexion,  yellow  spots — such  as  freckles — appear 
almost  black  in  the  picture.  Nevertheless,  these  dis- 
advantages can  be  obviated  by  employing  the  negative 
retouche  described  at  a  future  page.  On  the  other 
hand,  the  inoperative  property  of  yellow  light  has 
also  its  advantages  for  the  photographer.  It  permits 
him  to  prepare  the  sensitive  plates  in  a  subdued  light 
which  does  not  injure  them,  and  yet  suffer  his  eyes  to 
control  the  work.  If  the  plates  were  sensitive  to  all 
kinds  of  light,  it  would  be  necessary  to  prepare  the 
plates  in  absolute  darkness,  which  would  be  very  incon- 
venient. 

.Tha.-firsL.op£fatign  Required  in  j^eparing  a  sensitive 
plate — an  operation  which  requires  great  care — is  the 
cleaning  of  the  glass.  The  slides,  after  being  cut  by 
the  diamond,  are  placed  some  hours  in  a  corrosive  .fluid 
— nitric  acid — and  by  this  means  all  impurities  adhering 
to  the  surface  are  destroyed. 


40  THE    CHEMISTRY   OF   LIGHT. 

The  acid  adhering  to  it  is  removed  by  washing,  and  the 
plate  is  then  dried  with  a  clean  cloth.  To  the  uninitiated 
it  would  then  appear  perfectly  clean,  but  the  photo- 
grapher subjects  it  to  further  polishing,  by  rubbing  with  a 
few  drops  of  spirits  of  wine ;  or,  still  better,  of  ammonia. 
Each  touch  with  the  finger  or  rub  of  the  sleeve  of  the 
cleaned  surface,  each  drop  of  saliva  which  might 
chance  to  escape  from  the  mouth  in  coughing,  would 
spoil  the  polished  surface;  nay,  even  the  atmospheric 
air  produces  with  time  disadvantageous  effects.  If  a 
cleaned  plate  is  left  only  twenty-four  hours  in  the  air,  it 
gradually  attracts  its  exhalations,  and  another  cleansing 
is  rendered  necessary. 

The  cleaned  glass  is  saturated  with  collodion.  The 
collodion  itself  is,  as  we  know,  a  solution  of  gun-cotton 
in  a  mixture  of  alcohol  and  ether,  to  which  metallic 
iodine  and  bromine — for  instance,  iodide  of  potassium 
and  bromide  of  cadmium — have  been  added.  This  solu- 
tion must  also  be  produced  with  the  greatest  attention  to 
cleanliness ;  and,  in  order  to  preserve  the  purity  of  the 
materials  employed,  the  mixture  must  be  allowed  to 
stand  a  long  time,  and  the  sediment  carefully  cleared  of 
all  fluidity.  The  coating  of  a  plate  with  collodion  is  an 
affair  of  dexterity,  and  only  succeecl^Vith  those  who 
have  witnessed  the  process  and. after  s%ii'e  practice. 

It  is  usual  to  hold  the  corner  .of  the  plate  with  the 
hand  and  to  pour  over  the  centre  of  it  a  circular  mass  of 
the  thick  fluid,  and  then  to  allow  this  to  flow  to  all  of 
the  four  corners  by  a  gentle  inclination  of  the  plate  in 
different  directions,  and  thus  ultimately  to  let  the  fluid 
flow  off  at  one  of  the  corners. 

A  considerable  part   of  the   fluid  originally  poured 


THE   NEGATIVE   PROCESS.  41 

upon  it— that  is,  nearly  half — remains,  and  adheres  to 
the  plate. 

In  the  process  of  flowing  off,  streaks  are  usually 
formed,  which  would  likewise  spoil  the  picture;  and 
therefore  the  plate,  whilst  being  drained,  must  be  con- 
stantly kept  in  motion  until  the  last  drop  has  run  off. 
The  fluidity  stiffens  into  a  soft,  moist,  spongy  film. 
At  the  moment  when  this  thick  film  has  become  stiff, 
the  plate  must  at  once  be  immersed  in  the  solution  of 
silver  (silver  bath) . 

And  now  a  somewhat  unusual  action  of  the  fluids 
takes  place,  for  the  film  of  collodion  repels  like  fat 
the  watery  solution  of  silver,  and  a  steady  agitation  of 
the  plate  in  the  solution  is  necessary  in  order  to  make 
the  solution  adhere  to  the  plate. 

This  mechanical  operation  is  accompanied  simulta- 
neously by  a  chemical  change.  The  salts  of  iodine  and  of 
bromine  that  exist  in  the  collodion  film  change  their 
properties  with  nitrate  of  silver,  and  give  birth  to  iodide 
and  bromide  of  silver,  and  to  nitric-acid  salts.  This 
iodide  and  bromide  of  silver  colours  the  film  yellow; 
and  it  is  only  now  that  the  plate  is  prepared  which 
serves  as  the  ground  of  the  picture  about  to  be  painted 
by  the  light. 

All  of  these  operations  must  precede  the  taking  the 
photograph,  and  they  are  begun,  in  fact,  at  the  moment 
when  the  person  enters  the  atelier,  and  with  proper 
management  the  plate  is  prepared  before  the  arrange- 
ment forjtakingjhe_£ortrait  is  concluded.  . 

This  arrangement  is  a  labour  of  itself;  and  it  is  of  a 
genuinely  artistic  nature.  The  points  to  which  the 
photographer  has  to  attend  include  a  natural  and  yet 


42  THE    CHEMISTRY   OF   LIGHT. 

graceful  attitude  of  the  original ;  the  choice  of  the  side 
which  presents  the  most  advantageous  aspect;  the 
picturesque  arrangement  of  the  dress ;  the  removal  of 
inappropriate  objects  which  ought  not  to  appear  in  the 
picture ;  the  addition  of  those  that  are  suitable,  such  as 
a  table,  a  cabinet,  or  a  background;  lastly,  an  appro- 
priate direction  of  the  light.  Only  a  few  minutes  can 
be  devoted  to  these  arrangements,  for  persons  cannot 
endure  long  delays  or  experiments ;  and  the  plate  itsell 
only  lasts  a  short  time  in  the  sensitive  condition,  for  it 
is  damp  through  the  adhering  solution  of  silver,  which 
soon  dries  up,  and  the  plate  is  then  useless. 

When  the  exposure  to  the  light  has  been  accom- 
plished— a  process  during  which  the  person  being 
photographed  must  remain  perfectly  quiet — the  sensitive 
plate  is  brought  back  into  the  dark  chamber. 

For  the  purpose  of  transporting  the  plate,  which  must 
evidently  be  guarded  very  carefully  from  the  daylight, 
the  photographer  employs  a  little  flat  box  (Fig.  10), 
called  the  cassette,  whose  floor,  H,  and  cover,  D,  can  be 
drawn  out  and  closed.  In  the  corner  there  are  silver 
ledges  on  which  the  plate  lies ;  a  wedge  fastened  to  the 
upper  lid  keeps  them  in  their  place.  Thus  they  can  be 
easily  carried  in  the  closed  cassette  and  placed  within 
the  camera  obscura ;  there  its  ground-glass  slide  is  moved 
to  and  fro  until  the  picture  clearly  appears  upon  it. 
After  the  exposure  to  light  has  taken  place  the  plate  is 
taken  back  in  the  cassette  to  the  dark  chamber. 

And  now  follows  one  of  the  most  important  operations, 
the  development  of  the  picture.  Upon  the  plate  there  is 
as  yet  no  trace  of  a  picture  visible.  The  operation  of 
the  light  consists  in  quite  a  peculiar  change  of  the 


THE    NEGATIVE    PROCESS. 


43 


iodide  of  silver  which  forms  the  principal  constituent  of 
the  plate.  This  iodide  obtains  through  the  light  the 
property  of  attracting  pulverized  silver,  if  this  has  been 
precipitated  on  the  plate  in  any  shape.  This  precipitate 
is  produced  by  the  following  operation.  If  a  solution 
of  silver  is  mixed  with  a  very  diluted  solution  of  green 
vitriol,  there  results  by  slow  degrees  a  precipitate  of 
metallic  silver — not,  however,  as  a  green  shining  mass, 
but  as  a  grey  powder.  A  solution  of  silver  adheres 


Fig.  10. 

now  to  the  sensitive  plate  resulting  from  the  bath.  If 
after  this  a  solution  of  green  vitriol  is  poured  upon  it,  a 
silver  precipitate  is  also  occasioned,  and  the  picture  is 
seen  suddenly  to  make  its  appearance,  by  the  silver 
powder  adhering  to  the  part  exposed  to  light. 

The  features  of  a  portrait  that  are  first  visible  are  the 
lightest — the  shirt,  then  the  face,  and  lastly  the  black 
coat.  The  negative  thus  obtained,  however,  is  by  no 
means  completed  by  the  operation. 

The  picture  is  usually  too  attenuated  to  answer  in  the 


44  THE   CHEMISTRY   OF   LIGHT. 

positive  process  for  the  production  of  a  paper  impression 
with  the  help  of  light ;  for  the  production  of  such  an 
impression  results  from  the  light  shining  through  the 
transparent  places  of  the  negative,  and  colouring  dark 
the  places  underlying  them,  while  it  is  repelled  from 
the  parts  which  have  to  remain  black.  The  parts  in 
question  of  the  negative  must  be  sufficiently  transparent 
to  produce  this  effect. 

The  impression  must,  therefore,  be  more  strongly 
defined ;  and  this  takes  place  by  repeating  the  developing 
process.  A  mixture  of  green  vitriol  and  of  a  solution  of 
silver  is  poured  upon  the  picture,  and  a  silver  precipitate 
is  formed  again  on  it,  adhering  only  to  the  lines  of  the 
picture,  and  therefore  giving  these  an  intenser  colouring. 
If  the  plate  is  not  perfectly  clean  in  the  processes  of 
developing  and  defining,  silver  is  precipitated  upon  the 
dirt  stains  and  produces  spots.  After  the  defining  of  the 
picture,  or  the  so-called  fortifying  process,  has  been  com- 
pleted, it  is  only  necessary  to  remove  the  iodide  of  silver, 
which  diminishes  the  transparency  of  the  clear  parts  of 
the  plate.  Then  a  solution  of  hypo-sulphite  of  soda 
is  poured  on  the  plate.  This  salt  has  the  property 
of  dissolving  insoluble  salts  of  silver,  so  that  the  iodide 
of  silver  vanishes  under  the  influence  of  this  solution. 
This  is  the  fixing  process.  Lastly,  the  plate  is  washed 
and  dried.  If  it  be  borne  in  mind  that  all  these  different 
operations  are  performed  on  a  little  film,  liable  to  be 
injured  by  the  least  contact  of  any  kind,  it  is  not  to  be 
wondered  at  that  in  treating  matters  of  such  a  delicate 
nature  the  inexperienced  beginner  has  to  destroy  so 
many  coatings  before  a  proper  one  is  prepared. 

Even  when  dried,  the  picture  is  very  liable  to  injury ; 


THE   NEGATIVE    PKOCESS.  45 

and  therefore  photographers,  in  order  to  protect  it, 
cover  it  with  a  varnish,  that  is,  with  a  solution  of  a 
resinous  nature,  such  as  shell-lac  and  orpiment,  or  red 
arsenic  in  spirits  of  wine.  The  fragile  glass  negative  is 
thus  brought  to  completion. 

This  sketch  of  the  operations  which  a  photographer 
is  obliged  to  carry  on  in  order  to  produce  a  negative,  is 
sufficient  to  show  that  photography  is  a  more  difficult 
art  than  some  persons  imagine,  and  that  it  requires 
something  more  than  the  opening  and  shutting  of  a 
lid. 

The  chief  requisite  for  the  success  of  these  operations 
is  routine,  that  is,  the  unfailing  accuracy  obtained  in 
practising  each  part  of  the  process.  Faults  that  are 
made  in  any  particular  operation  of  the  process  are, 
as  a  general  rule,  irremediable ;  and  therefore  it  is 
absolutely  essential  to  avoid  them. 


46  THE    CHEMISTRY   OF   LIGHT. 


CHAPTEE  VI. 

THE  POSITIVE  PROCESS. 

Character  of  the  Negative — Departure  from  Nature — Negative  EetoucTie 
— Production  of  Sensitive  Paper — Striking  off  Impressions — Toning 
with  Chloride  of  Gold — Fixing — Cause  of  Fading — Quantity  of  Silver 
in  the  Picture — Toning  down  of  Photography. 

IN  the  preceding  chapter  we  have  become  acquainted 
with  the  production  of  a  negative  from  nature.  However 
interesting  such  a  negative  might  be,  nevertheless,  it 
could  not  satisfy  the  purchaser  of  a  portrait,  because  it 
showed  everything  reversed.  The  white  face  it  made 
black,  and  the  black  coat,  light.  No  one  would  hang 
up  on  his  wall  a  picture  representing  him  as  a  Moor. 
It  was  therefore  necessary  to  obtain  a  positive  impres- 
sion from  this  negative.  We  have  already  learnt  how 
this  is  effected  in  the  chapter  on  the  "  Licht-paus  " 
process.  It  is  the  old  Talbot  method  that  is  here  em- 
ployed. But  we  must  still  mention  some  very  important 
collateral  operations  which  are  of  high  significance  in 
modern  photography. 

The  camera,  the  negative  process,  and  the  photo- 
grapher who  knows  how  to  manipulate  intelligently, 
no  doubt  produce  a  negative  which,  laid  over  sensitive 
paper  and  exposed  to  the  light,  yields  a  positive ;  but 


THE    POSITIVE    PROCESS.  47 

although  this  positive  is  very  faithful  in  the  delineation  of 
figures — that  is,  of  their  outline — it  yet  presents  marked 
departures  from  nature.  It  is  especially  evident  that 
the  relations  of  light  and  shade  are  by  no  means 
correctly  given.  In  general  the  light  parts  appear  too 
light,  the  dark  parts  too  dark — as,  for  example,  the  folds 
in  a  dress,  the  skin,  and,  moreover,  the  shading  under 
the  eyes  and  chin.  When  photographers  knew  nothing 
of  art,  these  defects  were  taken  as  a  matter  of  course. 
People  protested  that  photography  was  correct  because 
nature,  through  photography,  was  herself  the  artist. 
But  in  this  conclusion  the  co-operation  of  the  photo- 
grapher was  overlooked. 

No  doubt  nature  that  is,  the  object  to  be  taken — 
makes  an  impression  upon  the  plate,  by  the  light  issuing 
from  it ;  but  an  impression  of  light  is  not  a  picture- 
it  is  indeed,  of  itself,  invisible ;  nay,  more,  the  strength 
of  the  impression  of  light  is  entirely  at  the  discretion  of 
the  photographer,  who  can  make  it  weak  or  intense 
by  a  greater  or  less  exposure.  There  is  no  rule  which 
determines  the  length  of  time  a  photograph  has  to  be 
exposed  to  the  light. 

The  fact  is.  that  nature,  properly  speaking,  only 
determines  the  outline  of  the  picture,  while  the  relations 
of  light  and  shade  depend  partly  on  those  distinctions 
of  nature,  and  partly  on  the  good  pleasure  of  the 
photographer. 

The  print  or  impression  of  the  light  is  developed; 
hence  it  becomes  visible,  and  finally  the  deyeloped  picture 
is  brought  more  strongly  out.  By  this  means  the  photo- 
grapher can  at  his  option  increase,  and  even  exaggerate, 
the  contrasts  of  light  and  shade. 


48  THE   CHEMISTRY   OF  LIGHT. 

If  the  negative  is  carefully  compared  with  the  original, 
we  shall  find  that  many  dark  parts  have  not  appeared 
at  all,  because  the  exposure  was  too  limited  for  them  to 
produce  an  impression  upon  the  plate ;  others  have  ap- 
peared, but  too  indistinct.  On  the  other  hand,  very 
clear  parts — for  instance,  the  shirt-collar — have  an  excess 
of  clearness  and  whiteness,  and  the  needlework  upon  it 
is  invisible  because  the  time  of  exposure  was  too  short. 
In  the  case  of  long  exposures  it  is  often  remarked  that 
clear  parts  differing  little  in  colour  are  entirely  con- 
founded, that  is  to  say,  form  a  single  white  patch. 

Moreover,  the  accessories  which  a  painter  would  un- 
doubtedly omit,  such  as  warts,  pockmarks,  little  hairs, 
are  all  as  clearly  defined  as  the  principal  features ;  and 
thus  the  negative  is  neither  a  correct  nor  an  agreeable 
repetition  of  .the  reality,  but  produces  in  the  positive  a 
picture  which  shows  considerable  departures  from  nature, 
and  is  often  inaccurate  by  giving  too  much  prominence 
to  accessories. 

In  the  first  period  of  photography  these  departures 
were  overlooked.  Every  one  was  content  to  possess  a 
portrait  which  at  least  showed  the  outlines  correctly; 
and  what  was  defective  in  the  negative  it  was  sought  to 
atone  for  through  the  retouch  of  the  positive.  But  this 
retouch  rendered  the  picture  dear ;  and  as  it  began  to  be 
the  custom  to  order  pictures  by  the  dozen,  the  endeavour 
was  made  to  evade  this  labour,  which  had  to  be  applied 
to  each  individual  picture,  by  carrying  it  out  in  the 
negative. 

A  single  touched-up  negative  gave  hundreds  of  un- 
exceptionable impressions  which  did  not  require  to  be 
retouched,  and  thus  the  negative  retouche  became  the 


THE   POSITIVE   PROCESS.  49 

first  and  most  important  operation  to  produce  a  faithful 
and  agreeable  picture.  The  essential  characteristic  of 
this  negative  retouche  consists  in  entirely  covering  several^ 
parts.  For  example,  the  freckles  and  warts  in  the  clear 
negative  are  entirely  removed  by  the  pencil,  or  Indian 
ink.  Other  parts — for  example,  the  too  delicate  details 
of  the  hair — are  more  defined  by  pencil  strokes.  Many 
shadows — for  example,  the  wrinkles  in  the  face — are 
softened  off  by  slight  touches  of  Indian  ink.  This  labour 
must  always  be  carried  out  with  the  thought  that  all 
which  the  painter  draws  on  the  negative  with  his  black- 
lead  pencil  will  appear  the  opposite ;  that  is,  light  in 
the  positive. 

It  is  requisite,  therefore,  for  the  negative  retouche  to 
have  a  thorough  knowledge  of  working  with  lead-pencil  and 
Indian  ink,  to  render  the  different  shades  in  the  positive 
process.  The  best  draughtsman  and  painter  is  on  that 
account  still  far  from  being  able  to  retouch  a  negative. 

It  is  to  be  remarked  that  the  negative  retouche  may, 
under  certain  circumstances,  go  too  far.  By  covering 
each  wrinkle  he  can  make  an  old  face  young ;  he  can 
beautify  an  ugly  original  by  cutting  away  a  hump  on 
the  back,  or  other  abnormal  growths ;  and  these  tricks 
are  often  put  into  requisition  for  the  vanity  of  sitters, 
and  are  dearly  paid  for. 

Plate  VI.  (p.  245)  represents  two  portraits  of  the  same 
person,  one  after  a  retouched  negative,  the  other  after 
a  negative  that  had  not  been  retouched.  They  repre- 
sent a  celebrated  singer  (Mdlle.  Artot) ;.  the  spots  on 
the  skin  and  the  dark  shadows,  on  the  picture  which  is 
not  retouched,  are  clearly  to  be  seen,  while  in  the  re- 
touched one  they  are  not  visible. 


50  THE    CHEMISTRY   OF   LIGHT. 

In  many  cases  the  negative  retouche  is  a  concession 
to  human  vanity,  but  this  is  by  no  means  always  the- 


As  already  explained  above,  photography  does  not 
always  reflect  correctly  the  natural  colours.  Yellow 
often  becomes  black,  and  blue,  white.  Therefore,  in 
producing  a  picture  of  brighter  hues,  photography  is 
often  very  deficient  in  the  reproduction  of  their  tones. 
Then  the  negative  retouche  is  a  powerful  aid  to  correct 
this  fault,  and  through  this  alone  have  photographs 
taken  from  oil  paintings  attained  their  present  perfection. 
We  will  treat  of  this  subject  in  a  future  chapter.  Let 
us  now  consider  the  operations  of  the  positive  processes. 
The  first  operation  is  the  production  of  the  sensitized 

paper.  A  piece  of  paper 
coated  with  white  of  egg 
and  moistened  with  a  solu- 
tion of  kitchen  salt  is  laid 
in  a  cup  with  a  solution  of 
silver.  The  paper  floats 
upon  the  liquid,  it  sucks 
it  up,  and  chloride  of 
silver  is  formed,  through 
decomposition  with  the 
kitchen  salt.  At  the  end 

of  a  minute  the  paper  is  taken  out  of  the  silver  solution. 
The  wet  paper  is  but  slightly  sensitized  ;  it  becomes 
fully  sensitized  only  after  being  dried.  The  dry  paper, 
saturated  with  chloride  and  nitrate  of  silver,  is  then 
pressed  together  in  the  copper  frame  (Fig.  11),  which  is 
similar  to  the  one  described  at  a  previous  page.  Then 
the  whole  is  exposed  to  the  light.  The  same  process 


THE   POSITIVE   PROCESS.  51 

ensues  which  we  have  described  in  the  chapter  on 
"  Licht-paus  paper  " ;  the  light  shines  through  the  clear 
places  of  the  negative  and  colours  dark  the  paper  lying 
under  them,  but  the  paper  under  the  dark  places  of 
the  negative  remains  white,  while  it  assumes  a  slight 
colour  under  the  half-tones.  In  this  manner  a  faithful 
positive  reprint  of  the  negative  is  produced,  presenting 
a  beautiful  violet-brown  tint.  We  know  from  the 
description  of  the  licht-paus  process,  that  this  reprint 
would  not  stand  the  light  long,  because  the  paper  is 
still  sensitive  to  light.  The  salts  of  silver  contained  in 
it  must  be  removed  if  the  impression  is  to  be  made 
lasting.  To  this  end  a  solution  of  hypo-sulphite  of  soda 
must  be  employed.  If  the  impressions  be  plunged  in 
this  solution,  they  become  durable  in  the  light ;  but,  un- 
fortunately, by  thus  dipping  them,  they  suffer  a  peculiar 
change  of  colour,  assuming  an  ugly  brown  tint.  This 
tint  is  no  injury  in  technical  and  scientific  pictures,  but 
detracts  greatly  from  portraits  and  landscapes ;  and  in 
order  to  give  these  a  more  agreeable  tint  before  fixing 
them,  they  are  plunged  into  a  diluted  solution  of  chloride 
of  gold.  This  process  is  called  toning  down. 

In  this  operation  a  part  of  the  gold  is  precipitated  on 
the  outlines,  giving  to  these  a  bluish  shade ;  and  now, 
after  plunging  it  into  a  solution  of  fixing  sodium,  the 
tone  of  the  picture  is  not  essentially  altered. 

The  picture  thus  produced  consists  partly  of  gold, 
partly  of  silver,  in  a  finely  powdered  state,  and  only 
requires  to  be  thoroughly  washed  in  ordor  to  establish 
its  durability.  If  this  washing  is  omitted,  small  particles 
of  fixing  sodium  holding  sulphur  in  suspension  remain 
behind,  and  these  become  decomposed  and  form  on  the 


52  THE    CHEMISTRY   OF   LIGHT. 

picture  yellow  sulphide  of  silver.  This -accounts  for  the 
fact  that  the  pictures  of  an  earlier  period,  when  from 
ignorance  of  these  results  this  thorough  washing  was 
neglected,  so  often  turned  out  pale  and  yellow. 

It  is  surprising  what  a  small  amount  of  silver  and 
gold  is  required  to  give  an  intense  colour  to  a  whole 
sheet  of  paper.  For  in  a  sheet  of  this  description — 
1*447  feet  by  1*546* feet— which  has  become  completely 
blackened,  there  are  only  1-3162  grains ;  whilst  in  a 
picture  of  this  size  there  are  only  0*075,  that  is,  about 
one-thirteenth  of  15*440  grains,*  and  in  a  carte  de  visite 
one-five-hundredth  of  15*440  grains!  of  silver, 

It  must  be  here  remarked  that  pictures  which  are 
fresh  when  printed,  pale  a  little  in  the  fixing  process; 
and  hence  the  photographer  usually  prints  these  darker 
than  they  ought  to  remain.  Accordingly,  the  printing 
process  requires  a  practised  eye,  simple  as  it  may 
appear. 

In  certain  cases  tricks  of  art  are  employed  to  produce 
agreeable  effects,  and  among  these  is  that  of  toning 
down.  Our  readers  are  no  doubt  well  acquainted  with 
portraits  •  on  a  white  ground,  the  outlines  of  which 
gradually  become  confounded  with  the  ground  tint  of 
the  picture.  This  effect  is  produced  in  a  very  simple 
manner  by  placing  what  is  called  a  mask  on  the  copying 
frame.  This  mask  is  a  piece  of  metal  or  cardboard 
(Fig.  12)  in  which  an  oval  hole  b  is  cut.  This  is  placed 
on  the  copper  frame  K  K,  so  that  the  part  of  the 
negative  which  shall  be  impressed  on  the  picture  lies 
perpendicularly  under  it.  This  part  is  then  affected 

*  One-thirteenth  of  a  gramme  =  1'187  grains. 
•f  One-five-hundredth  of  a  gramme  =  '031  grains. 


THE   POSITIVE  "PROCESS.  53 

perpendicularly  by  the  broad  bundles  of  light  S  S,  and 
intensely  coloured,  while  the  collateral  parts  lying  under 
the  mask  are  affected  only  by  the  small  patch  of  light, 
S'  S'.  Accordingly,  they  only  give  a  pale  reprint  on  the 
paper,  as  they  are  remote  from  the  margin  of  the  mask. 
Thus  a  gently  vanishing  margin  is  produced,  looking 
very  artistic,  and  yet  only  the  result  of  a  very  simple 
trick  of  art. 

The  picture  which  the  photographer  produces  in  the 
manner  now  described  only  requires  some  rectifying 
to  be  an  elegant  drawing-room  ornament.  It  is  cut  in 
a  regular  shape,  square  or  oval,  and  fastened  with  clean 
paste  to  white  cardboard,  and  finally,  after  drying,  and 
the  removal  of  little  blemishes,  by  slightly  touching  up 


Fig.  12. 

with  the  .paint  brush,  it  is  rendered  glossy  by  two  smooth 
steel  rollers,  and  receives  a  satin-like  surface. 

Certain  sizes  have  been  adopted  through  custom  by 
the  public.  Among  these  is  the  shape  of  what  is  called 
the  "carte  de  visite,"  and  " cabinet  size."  The  former 
is  rather  larger  than  an  ordinary  visiting  card.  The 
latter  is  two  and  a-half  times  as  large.  f 

The  carte  de  visite  was  introduced  at  Paris  by  Disderi, 
in  1858,  speedily  secured  admirers,  and  has  been 
diffused  over  the  whole  earth.  Even  chemical  photo- 


54  THE    CHEMISTRY   OF   LIGHT. 

graphers  prepare  photographs  in  the  form  of  cartes  de 
visite. 

The  carte  de  visite  and  the  cabinet  form — first  adopted 
in  England,  and  a  great  favourite  in  America — are  not 
confined  to  portraits,  but  also  employed  for  landscapes 
and  photographs  taken  from  oil  paintings.  Millions  of 
these  pictures  are  sold  every  year,  and  a  properly 
arranged  album  for  preserving  them  is  found  in  almost 
every  family. 

Photography  admits  of  such  small  forms  because  of  its 
fine  details,  but  it  is  by  no  means  confined  to  them.  It 
freely  admits  surfaces  that  take  in  a  portrait  of  life-size. 
The  production  of  the  latter  necessitates  a  peculiar 
process,  called  the  enlarging  process,  which  will  be 
treated  of  in  a  future  page. 


CHAPTEE  VII. 

LIGHT  AS  A  CHEMICALLY-OPERATIVE  AGENT. 

Theory  of  Photography — Nature  of  Light — Undnlatory  Theory — Semi. 
Tones— Mouldering  away  of  Red  Sulphuret  of  Arsenic — Chemical 
Decomposition  by  Light — Colours  and  Tones — Their  Vibrations — 
Refraction — Dispersion  of  Colours — The  Spectrum — Spectral-Lines — 
Invisible  Rays  —  Photographs  of  Lunar-Landscapes  —  Abnormal 
Photographic  Effect  of  Colours— Photography  of  the  Invisible. 

GOETHE  says,  "All  theory  is  grey,  and  the  golden- 
tree  of  life  green."  This  saying  has  often  been  mis- 
understood and  abused,  especially  by  those  too  lazy  to 
think ;  but,  faithful  to  its  true  meaning,  we  have  first 
treated  of  a  multitude  of  facts  from  life — that  is,  from  the 
history  and  practice  of  photography, — and  now  we  pro- 
ceed to  describe,  by  the  help  of  science,  how  and  why, 
not  the  golden  but  the  silver  tree  of  photography  becomes 
verdant,  blooms  and  bears  such  splendid  fruit. 

Two  sciences  join  ha<nd  to  accomplish  the  wonders  of 
photography.  One  is  Optics,  a  division  of  Physics,  and 
the  other  Chemistry.  "We  have  already  shown  that  they 
alone  are  inadequate  to  fulfil  the  requirements  for  the 
production  of  a  photograph.  ^Esthetical  claims  have 
to  be  considered  ;  and  thus  photography  unites  in  itself 
the  provinces  of  natural  science  and  of  the  fine  arts 


56  THE   CHEMISTRY  OF   LIGHT. 

which  seein  remote  and  incapable  of  union.  We  shall 
attend  first  to  the  optical  principles — that  is,  to  light — 
as  the*  force  which  occasions  the  chemical  changes  in 
photography.  We  shall  see  that  its  chemical  operations 
have  not  only  become  the  basis  of  our  art,  but  that  they 
have  played,  and  still  play,  a  still  more  important  part 
in  the  development  of  our  planet. 

We  are  aware  of  the  existence  of  sun,  moon,  and 
planets.  We  know  their  distance ;  nay  more,  we  know 
their  elements,  though  we  are  separated  from  them  by 
millions  of  miles. 

We  are  indebted  for  all  this  knowledge  to  light.  What 
is  light  ?  An  undulation  of  the  ether.  And  what  is  the 
ether?  An  infinitely  delicate  fluid,  which  fills  all  the 
space  of  the  universe,  and  undulates  like  all  fluid.  If  we 
throw  a  stone  into  water,  waves  are  produced — that  is, 
circles  or  rings,  or  hills  and  valleys,  are  formed ;  these 
appear  to  widen  out  from  a  centre,  and  as  they  extend 
become  gradually  less,  until  they  finally  disappear.  If 
several  little  stones  are  thrown  at  the  same  time  into 
the  water,  each  of  them  forms  its  own  system  of  undula- 
tions. These  intersect  each  other  in  the  most  compli- 
cated manner ;  and,  although  a  confusion  of  rings  takes 
place,  it  is  wonderful  that  none  of  them  disturbs  the  other, 
and  that  'each  circle  widens  out  regularly  from  its  own 
centre,  where  the  stone  fell  into  the  water.  (See  Fig.  13.) 

If  a  handful  of  sand,  which  contains  many  thousand 
grains,  is  thrown  into  water,  and  if  the  attention  be 
directed  to  the  undulations  of  a  single  grain,  it  will  be 
probably  remarked  that  this  one,  without  being  affected 
by  the  countless  other  waves,  widens  out  into  a  regular 
circle. 


LIGHT   AS   A    CHEMICALLY-OPEBATIVE   AGENT.  57 

These  undulations  are  one  of  the  most  remarkable 
movements  in  nature,  taking  place  not  only  in  water, 
but  in  the  air,  where  they  occasion  the  propagation  of 
sound. 

The  peculiar  feature  of  the  undulatory  movement 
consists  in  the  fluid  appearing  to  advance  without  really 
doing  so.  If,  sitting  on  the  side  of  a  sheet  of  water,  we 
see  an  undulation  approach,  it  appears  exactly  as  if  the 
particles  of  water  were  approaching  us  from  the  origin 
of  the  movement. 

It  is  easy  to  prove  that  this  is  an 
error  by  throwing  sawdust  or  a  piece 
of  wood  into  the  water.  It  dances 
up  and  down  upon  the  ripples  with- 
out moving  from  the  spot.  Indeed, 
the  undulation  is  itself  only  an  up 
and  down  motion  of  the  particles  of 
the  water,  and  this  movement  it  Fi£- 13- 

communicates  further  and  further  to  the  neighbouring 
particles  of  water. 

Exactly  in  the  same  manner  light  spreads  in  undula- 
tions from  a  luminous  body  through  the  ether  of  space 
in  all  directions.  The  movement  of  the  undulation 
we  call  a  ray  of  light.  We  perceive  it  as  soon  as  it 
reaches  our  eye,  whilst  the  vibrating  ether  strikes  our 
retina. 

Now,  we  know  that  the  undulations  of  tone  are  able 
to  set  other  bodies  in  motion.  If  the  A  or  second  string 
of  a  violin  is  struck,  the  A  string  of  a  piano  standing 
near  sounds  distinctly  with  it.  Nay,  even  if  the  damper 
-of  a  piano  is  raised  and  any  tone  made  to  vibrate, 
instantly  the  string  of  the  violin  sounds  which  has  a 


58  THE    CHEMISTRY   OF   LIGHT. 

similar  tone.  The  same  thing  happens  with  a  glass  bell 
of  the  same  tone.  There  are  people  even  who  can  make 
a  glass  break  by  a  shrill  tone  of  their  voice.  The  glass  is 
so  shaken  by  the  violent  undulations  of  the  air  that  it 
falls  to  pieces.  Tinder  such  circumstances,  it  need  not 
surprise  us  that  the  undulations  of  light  agitate  bodies 
so  forcibly  that  they  fall  to  pieces. 

Bed  sulphuret  of  arsenic  offers  the  most  remarkable 
example  of  this  kind.  This  is  a  beautiful  mineral  of  a 
ruby  red  colour,  in  the  form  of  splendid  crystals,  which 
consist  of  sulphur  and  arsenic.  If  a  crystal  of  this 
kind  be  exposed  for  months  to  the  light,  it  becomes 
pliant  and  falls  into  powder;  and  in  this  way  many 
very  fine  pieces  of  this  beautiful  mineral  have  been  lost 
in  the  mineralogical  museum  of  Berlin. 

This  is  only  a  mechanical,  and  not  a  chemical,  opera- 
tion of  light ;  but  it  gives  an  insight  into  its  chemical 
working.  Heat  occasions  chemical  decomposition  by 
extending  bodies,  and  thereby  removing  their  atoms  so 
far  apart  that  the  chemical  power  which  unites  them 
loses  effect,  and  the  component  parts  separate.  Thus 
the  oxide  of  mercury  is  by  heat  resolved  into  its  parts, 
mercury  and  oxygen. 

This  decomposition  is  effected  by  light  when  the 
atoms  of  a  body  are  agitated  by  its  undulations,  that  is 
to  say,  are  made  to  vibrate  ;  and  if  these  vibrations  are 
unequal,  a  separation  of  the  parts  takes  place,  and  the 
body  falls  to  pieces. 

The  undulations  of  light  are  not  a  fiction.  Not  only 
has  their  existence  been  ascertained,  but  their  size  has 
been  determined.  The  latter  is  extremely  minute,  but 
nevertheless  is  susceptible  of  measurement. 


LIGHT  AS  A  CHEMICALLY-OPE: 


The  waves  of  sound  and  the  waves  of  light  have  there- 
fore a  certain  analogy ;  and  as  there  are  different  tones 
in  music,  so  are  there  different  tones  in  light.  The 
number  of  tones  is  great.  The  simplest  piano  has  nine 
octaves,  and  there  are  tones  below  and  above  it.  But  the 
number  of  colours  is  small ;  only  seven  of  them  can  be 
distinguished — red,  orange,  yellow,  green,  blue,  dark 
blue,  and  violet, — the  well  known  colours  of  the  rain- 
bow. The  painter,  indeed,  contents  himself  with  three 
ground  tints — yellow,  blue,  and  red.  All  the  others  are 
the  result  of  their  mixture ;  and  the  larger  scale  of  colour 
of  the  painter  consists  not  of  simple  tones  of  colour,  but 
of  what  may  be  called  chords  of  colour. 

The  deep  tones  of  music  give  few  undulations,  the 
higher  tones  more.  For  example,  an  A  string  makes 
420  vibrations  in  a  second,  the  A  an  octave  lower  makes 
210,  the  great  A  105. 

In  light,  red  is  the  colour  which  gives  the  fewest 
vibrations ;  it  is  the  lowest  tone  in  colours,  and  violet  is 
the  highest,  giving  vibrations  twice  as  rapid  as  red.  With 
regard  to  tones,  we  know  that  they  all  spread  with  equal 
rapidity  in  the  air ;  if  this  were  not  the  case,  a  piece  of 
music  would  sound  in  the  -distance  as  the  most  disagree- 
able discord. 

It  is  the  same  in  the  kingdom  of  light — the  colours, 
without  exception,  are  propagated  through  the  ether 
with  equal  rapidity,  the  red  as  fast  as  the  violet.  But, 
whilst  the  reverberation  in  the  second  passes  over  only 
1024  feet  =  333  meters,  in  the  same  time  the  light 
hastens  42,000  miles,  and  the  deepest  tone  of  colour — 
red — traverses  in  a  second  420  billion  of  vibrations ;  that 
is  to  say,  a  million  times  million  as  many  as  the  tone 


60  THE    CHEMISTRY   OF   LIGHT. 

which  is  marked  in  music  with  a  bar  over  the  a,*  that  is, 


The  small  number  of  the  colour-tones  compared  with 
the  large  number  of  musical  tones  is  very  striking.  But 
the  fact  is,  that,  besides  the  seven  invisible  colours, 
there  exist  invisible  shades,  which  lie  partly  above  and 
partly  below  the  visible  colours. 

These  invisible  colour-tones  are  partly  disclosed  by 
the  thermometer,  which  reveals  the  lower  tones,  and 
partly  by  substances  sensitive  to  light.  For  it  is  remark- 
able that  the  colour-tones,  which  are  higher  than  the 
violet,  though  invisible,  have  a  powerful  chemical  effect. 

We  name  the  invisible  tones  of  colour  above  violet, 
ultra-violet,  and  those  beyond  red,  ultra-red. 

In  the  common  white  light  all  the  tones  of  colour  are 
found  together,  and  in  combination  they  excite  the 
feeling  of  whiteness ;  but  if  we  wish  to  consider  the  tones 
of  colour  separately,  we  must  part  them,  and  this  is 
done  by  the  help  of  a  prism. 

Every  polished  crown-glass  prism  causes  the  rays 
seen  through  it  to  appear  like  a  rainbow  streak  con- 
taining the  primitive  colours  we  have  named  above. 
This  separation  of  the  colours  in  the  prism  takes  place 
by  refraction. 

If  a  ray  of  light  passes  from  one  transparent  body  to 
another,  it  is  deflected  from  its  rectilinear  direction,  and 
this  deflection  is  named  refraction. 

*  We  may  here  remark  that  the  tone  a  is  not  everywhere  the  same. 
The  a  of  the  Berlin  Opera  is  the  highest ;  it  has  437  vibrations, — the 
Italian  Opera  at  Paris  only,  on  the  contrary,  424  vibrations.  We  have 
adopted  for  the  sake  of  simplicity  a  round  number,  420. 


LIGHT   AS  A   CHEMICALLY-OPERATIVE   AGENT.  61 

For  example,  if  the  ray  a  n  (Fig.  14)  sirikes  a  watery 
surface,  it  does  not  continue  in  its  original  direction  a  n, 
but  in  the  direction  n  b.  If  at  the  point  n,  where  the 
ray  falls  into  the  water,  a  perpendicular  line  n  d  be 
raised,  this  is  the  plunab  line ;  and  the  rule  is,  that  if  a 
ray  passes  from  a  thinner  medium  (for  example,  air)  into 
a  thicker  one,  it  approaches  the  plumb  line,  for  n  b  is 
evidently  nearer  to  the  plumb  line  than  n  a.  It  is 
otherwise  if  a  ray  passes  from  a  denser  to  a  thinner 
medium, — for  instance,  from  glass  into  air, — then  the 
ray  n  b  departs  from  the  plumb  line  n  d  ;  that  is,  the 
angle  which  it  makes  with  the  plumb  line  after  refraction 
is  greater  than  the  angle  which  it  makes  with  it  before. 

Now,  it  is  a  remarkable  fact  that  the  light  of  unequal 
shades  of  tone  is  refracted  also  unequally. 

If  a  bundle  of  white  sun's  rays 
is  suffered  to  fall  on  a  piece  of 
glass,  the  violet  rays  are  deflected 
in  a  greater  degree  than  the  blue 
rays,  the  blue  more  than  the 
green,  yellow,  and  red ;  and  the 
result  of  this  is  that  the  white 
bundle  of  rays  is  decomposed  into 
a  rainbow  -  coloured  fan,  violet,  Fl°- 14* 

indigo,  blue,  green,  yellow,  orange,  and  red. 

This  phenomenon  is  the  cause  of  the  rainbow.  If  a 
ray  a  falls  on  a  drop  of  water  (Fig.  15),  it  is  refracted 
and  at  the  same  time  divided  into  a  coloured  fan,  which 
is  reflected  from  the  lower  part  of  the  drop,,  suffers  again 
a  refraction  and  dispersion  of  colour  Z>,  and  issues  as  a 
broad  bundle  of  colour.  In  open  daylight  this  cannot 

be   clearly  seen,  because  our  eyes  are  dazzled  by  the 

4 


62 


THE    CHEMISTRY   OF   LIGHT. 


Fig.  15. 


clear  light  surrounding  them.  In  order  to  observe  the 
pure  colours  of  the  spectrum,  it  is  best  to  place  it  in  a 
darkened  room,  in  which  the  light  is  allowed  to  enter 
only  through  a  small  slit  (b  Fig.  16). 

The  prism  s  is  placed  on   a 
line  in  front  of  the  chink,  when 
the  colours  of  the  spectrum  are 
clearly  seen  upon  the   opposite 
wall.     If  the  chink  is  sufficiently 
narrow,  a  row  of  dark  lines  is 
observed    within   it,   which    cut 
through     the     coloured     stripes 
perpendicularly. 
These  lines  were  first  seen  by  Wollaston,  studied  more 
exactly  by  the  celebrated  Frauenhofer,  and  called  after 
him  Frauenhofer's  lines. 

The  lines  are  al- 
ways found  on  the 
same  spot,  so  that 
they  can  be  con- 
sidered as  natural 
music  lines,  upon 
which  the  scale  of 
colour  is  written  ; 
and  as  the  music 
lines  serve  for  the 
recognition  of  the 


Fig.  16. 


musical  tones,  so  do  the  lines  of  the  spectrum  indicate 
the  exact  places  of  the  scale  of  colour. 

If  we  use  the  term  green  in  the  spectrum,  this  would 
be  a  very  vague  designation ;  whereas  by  presenting  the 
line  on  the  spectrum  in  which  green  is  found,  its  place 


LIGHT   AS   A   CHEMICALLY-OPERATIVE   AGENT.  63 

is  at  once  made  known.  To  this  end  certain  character- 
istic names  were  given  to  the  lines  by  Frauenhofer, 
which  he  indicated  by  letters  ;  a  certain  line  in  the  red 
he  called  A,  another  in  the  yellow  D,  one  in  the  violet 
H9  and  H'.  As  the  number  of  lines  reaches  several 
thousand,  these  letters  do  not  suffice  to  indicate  them 
all.  (See  Fig.  17.) 

The  lines  thus  named  are  found  especially  in  the  sun- 
light ;  the  light  of  other  stars  commonly  shows  other  lines. 
The  light  from  artificial  sources  does  not  show  dark, 
but  bright  lines ;  a  flame  coloured  yellow  with  kitchen 
salt  shows,  for  example,  a  very  characteristic  line  in 
the  yellow ;  a.  burning  magnesium  wire  shows  more  blue 
and  green  lines. 

The  situation  of  these  lines  agrees  exactly  with  that 
of  certain  dark  lines  in  the  spectrum.  For  example,  the 
yellow  line  in  a  flame  coloured  with  kitchen  salt  exactly 
coincides  with  line  D  in  the  spectrum.  The  green 
lines  in  a  flame  of  magnesium  coincide  exactly  with 
lines  E  C  in  the  spectrum. 

A   B  d        D  Eb        F  a  H 


Fig.  17. 

This  remarkable  coincidence  led  to  the  surmise  that 
the  lines  in  the  sun's  solar  spectrum  might  owe  their 
existence  to  the  same  substances  that  'produced  the 
coinciding  lines  in  earthly  flames.  Kirchhoff  converted 
this  surmise  into  a  certainty,  and  was  thus  able  to 
determine  from  the  lines  in  the  solar  spectrum  the  sub- 


64  THE    CHEMISTRY   OF   LIGHT. 

stances  present  in  the  red-hot  body  of  the  sun,  and  thus 
to  demonstrate  the  chemical  composition  of  a  star  dis- 
tant more  than  90  millions  of  miles  (spectrum  analysis). 

But  the  spectrum  contains  still  other  numbers,  which 
have  not  been  discerned  by  the  human  eye,  but  by  the 
photographic  plate. 

If  a  sensitized  plate  be  exposed  to  the  operation  of  the 
spectrum,  it  is  observed  that  red  and  yellow  make 
only  a  very  feeble  impression  upon  it.  Light  blue  pro- 
duces more  effect,  but  dark  indigo  and  violet  the  most ; 
and  in  the  space  where  no  rays  can  be  perceived  by  our 
eyes,  a  distinct  impression  is  produced,  and  extends 
beyond  violet  for  a  space  almost  as  long  as  the  visible 
part  of  the  spectrum. 

From  this  fact  the  existence  of  the  ultra-violet  rays 
was  ascertained.  Accordingly,  the  retina  of  our  eye  and 
the  photographic  plate  show  an  entirely  different  sus- 
ceptibility. Our  eye  is  affected  most  powerfully  by 
yellow  and  green.  These  colours  appear  to  us  the 
clearest,  while  the  photographic  plate  is  not  at  all 
affected  by  them ;  but  it  receives  powerful  impressions 
from  indigo  and  violet  rays,  which  appear  dark  to 
our  eye,  and  even  from  rays  which  to  our  eyes  are 
invisible. 

Therefore  it  is  natural  that  photography  should 
represent  many  objects  in  a  false  light.  Further  back 
we  called  attention  to  the  fact  that  photography  is  much 
less  sensitive  to  feebly  lighted  objects  (objects  in  shadow). 
This  is  most  clearly  seen  in  the  fact  that  the  eye  can 
easily  perceive  objects  in  the  moonlight,  which  is  200,000 
times  weaker  than  that  of  the  sun  ;  whereas  the  photo- 
graphic plate  of  a  lunar  landscape  is  not  able  to  produce 


LIGHT   AS   A    CHEMICALLY-OPERATIVE   AGENT.  65 

any  picture  at  all.  The  photographic  lunar  landscapes 
sometimes  offered  for  sale  have  been  taken  in  the  day- 
light and  copied  very  darkly,  so  that  they  produce  the 
effect  of  moonlight.  These  pictures  are  very  popular 
at  Venice. 

This  small  susceptibility  of  the  photographic  plate  to 
feeble  light  explains  the  reason  why  shadows  in  photo- 
graphy are  generally  too  dark.  To  these  defects  must 
be  added  the  false  working  of  light, — blue  generally 
works  clear,  yellow  and  red  work  like  black.  The 
yellow  freckles  appear  therefore  in  a  picture  as  black 
spots,  and  a  blue  coat  becomes  perfectly  white.  Dark 
blue  flowers  on  a  light  yellow  ground  produce,  in  photo- 
graphy, light  flowers  on  a  dark  ground.  Bed  and  also 
fair  golden  hair  become  black.  Even  a  very  slight 
yellow  shade  has  an  unfavourable  effect.  A  photograph 
from  a  drawing  is  often  blemished  by  little  ironmould 
specks  in  the  paper  invisible  to  the  eye.  These  specks 
frequently  appear  as  black  points.  There  are  faces  with 
little  yellow  specks  that  do  not  strike  the  eye,  but  which 
come  out  very  dark  in  photography.  A  few  years  ago  a 
lady  was  photographed  at  Berlin,  whose  face  had  never 
presented  specks  in  photography.  To  the  surprise  of 
the  photographer,  on  taking  her  portrait  specks  appeared 
that  were  invisible  in  the  original.  A  day  later  the 
lady  sickened  of  the  small-pox,  and  the  specks  at  first 
invisible  to  the  eye,  became  then  quite  apparent.  Photo- 
graphy in  this  case  had  detected  before  the  human  eye 
the  pock-marks  very  feebly  tinged  yellow,. 

In  the  photographs  of  paintings,  such  abnormal  work- 
ings of  colour  are  still  more  evident,  and  can  only  be 
removed  by  appropriate  negative  retouches. 


66  THE    CHEMISTRY  OF  LIGHT. 

It  is  proper  to  observe,  however,  that  by  no  means  all 
shades  of  blue  become  light  in  photography.  For 
example,  indigo  forms  an  exception,  appearing  as  dark  as 
in  nature,  and  this  is  shown  in  the  photographs  of  the 
uniforms  of  Prussian  soldiers.  The  reason  of  this  is,  that 
indigo  contains  a  considerable  amount  of  red.  On  the 
other  hand,  cobalt  blue  and  ultramarine  produce  almost 
the  effect  of  white.  Again,  cinnabar  red  works  dark,  also 
English  red ;  whereas  madder  red,  which  contains  blue, 
becomes  very  light.  Chrome  yellow  becomes  much 
lighter  than  Naples  yellow,  Schweinfurt's  green  becomes 
lighter  than  cinnabar  green.  No  one  of  our  pigments 
contains  a  perfectly  pure  spectrum  colour,  but  consists 
always  of  a  mixture  of  different  colours,  and  therefore  is 
essentially  modified  by  photographic  operations. 

If  the  effect  of  the  colours  of  the  spectrum  on  photo- 
graphic plates  is  more  narrowly  examined,  it  is  observed 
that  indigo  produces  the  greatest  impression.  Never- 
theless, the  differently  sensitized  photographic  prepara- 
tions offer  somewhat  different  results  in  this  respect. 
Chloride  of  silver  is  most  sensitive  to  violet,  but  non- 
sensitive  to  blue.  Bromide  of  silver  is  also  sensitive  to 
green,  and  iodide  of  silver  only  to  violet  and  indigo. 
Mixtures  of  iodide  and  bromide  of  silver  are  only  sensitive 
to  blue  and  green.  The  writer  of  this  work  succeeded,  in 
the  end  of  1873,  in  making  photographic  plates  sensitive 
even  to  those  colours  that  were  before  considered  to  be 
inoperative,  i.e.  yellow,  orange,  and  red.  He  found  that 
if  certain  coloured  substances  that  absorb  light  were 
added  to  bromide  of  silver,  which  is  by  itself  too  little 
sensitive  to  green,  the  sensitiveness  of  this  bromide  to 
green  is  considerably  increased.  In  like  manner,  if 


LIGHT   AS  A   CHEMICALLY-OPERATIVE   AGENT.  67 

coloured  substances  absorbing  yellow  or  red  light  are 
added  to  it,  they  make  bromide  of  silver  sensitive  to 
yellow  and  red  light.  After  this  discovery,  we  may  hope 
that  the  difficulties  attending  the  taking  of  coloured 
objects  may  be  soon  overcome. 

Mention  has  often  been  made  of  the  photography  of 
the  invisible.  The  cases  already  recorded  of  the  photo- 
graphs of  invisible  pock-marks  belong  to  this.  But  the 
photography  of  an  invisible  quinine  writing  is  especially 
understood  by  the  term  photography  of  the  invisible. 
If  a  writing  is  made  on  paper  with  a  concentrated 
solution  of  sulphate  of  quinine,  the  result  is  scarcely 
visible.  If  this  is  photographed,  it  appears  black  and 
plainly  visible  in  the  picture.  The  sulphate  of  quinine 
has  the  property  of  lowering  the  tone  of  violet,  of  ultra- 
violet and  blue  rays ;  that  is,  of  converting  them  into 
rays  of  less  refraction  and  of  less  chemical  effect ;  there- 
fore the  light  issuing  from  quinine  produces  little  or  no 
effect,  and  the  written  characters  become  black. 

This  property  of  the  sulphate  of  quinine  serves  also  to 
make  ultra-violet  rays  visible.  If  a  piece  of  paper  that 
has  been  rubbed  with  sulphate  of  quinine  is  held  in  the 
spectrum,  the  originally  invisible  ultra-violet  part  of  the 
spectrum  is  seen  to  shine  in  the  bluish  green  light. 

Other  substances  produce  this  effect,  such  as  uranite, 
Devonshire  spar  (fluor),  and  therefore  this  property  has 
received  the  name  of  fluorescence. 


68  THE    CHEMISTRY   OF  LIGHT. 


CHAPTEE  VIII. 

CHEMICAL  EFFECT  OF  DIFFEEENT  SOUECES  OF  LIGHT. 

Artificial  Light— Magnesium  Light — Lime  Light — Electric  Light — 
Eepresentation  of  Subterranean  Places  by  Eeflected  Sunlight — 
Chemical  Intensity  of  Sunlight  and  of  the  Blue  Sky  Light — Breath, 
ing  of  Plants  under  the  Influence  of  Light  —Effect  of  Light  in  the 
History  of  the  Development  of  the  Earth  and  in  the  Economy  of 
Nature. 

FROM  the  facts  explained  in  the  foregoing  chapter,  it 
follows  that  chemical  effects  are  chiefly  produced  by  the 
ultra-violet,  violet,  and  blue  rays.  It  is  therefore  evident 
that  a  light,  from  whatever  source,  will  produce  chemical 
effects  with  an  intensity  proportioned  to  the  amount  of 
these  rays  it  contains. 

Lamplight,  gas  and  petroleum  light  are  very  poor  in 
such  rays.  Therefore  these  operate  only  feebly  on  the 
photographic  plate,  and  photographers  can  prepare 
their  sensitive  plates  in  a  subdued  lamplight. 

This  is  also  done  frequently  in  the  day  by  allowing 
the  light  to  pass  through  yellow  glass. 

The  white  Bengal  light  of  arsenic,  the  flames  of  the 
blue  Bengal  light,  and  those  of  burning  sulphur,  produce 
a  much  more  powerful  chemical  effect.  The  latter 
possesses  only  a  small  illuminating  power,  because  it 


CHEMICAL  EFFECT  OF  DIFFERENT  SOURCES  OF  LIGHT.      69 

contains^vellow  and  red  rays,  emitting  little  light ;  but, 
on  the  other  hand,  it  is  rich  in  blue  and  violet.  Photo- 
graphs have  been  actually  taken  by  help  of  these 
flames. 

But  the  above  are  greatly  surpassed  by  the  effect  of 
the  lime,  the  magnesium,  and  electric  lights.  The  mag- 
nesium light  is  very  simply  produced  by  the  burning  of 
magnesium  wire* 


Fig.  18. 

Magnesium  is  a  metal  which  forms  the  chief  com- 
ponent part  of  magnesia.  Magnesia  is  nothiijg  but 
magnesium  rust ;  that  is,  a  combination  of  magnesium 
with  oxygen. 

If  magnesium  wire  is  burned,  it  combines  at  a  red 
heat  with  the  oxygen  of  the  air,  precipitating  the  oxide 
of  magnesium.  The  magnesium  light  is  very  convenient 
in  its  application.  An  ounce  of  magnesium  wire, 


70 


THE    CHEMISTRY   OF   LIGHT. 


sufficient  for  fifteen  to  thirty  experiments,  can  be  easily 
carried  in  the  pocket.  But  the  general  use  of  the 
light  is  impeded  by  the  price  of  the  metal  (five  groschen 
the  gramme  *)  and  by  the  smoke  which  it  emits.  The 
writer  of  this  book  has  repeatedly  employed  it  with 
success  in  taking  the  sculptures  in  the  sepulchral  monu- 
ments of  Egypt.  When  burning  the  magnesium  wire, 
Solomon's  lamp  is  used  (Fig.  18).  This  consists  of  a 
round  vessel  K,  upon  which  the  wire  is  coiled,  a  watch- 
work  G,  which  conducts  the  wires  by  means  of  cylinders 
through  the  pipe  R,  at  the  top  /  of  which  the  wire  is 
lighted.  The  apparatus,  by  the  concave  mirror  O, 
throws  back  the  light  as  a  parallel  bundle  of  rays. 


Fig.  19. 

By  means  of  the  handle  H,  the  lamp  with  its  bundle 
of  rays  can  be  turned  in  any  direction,  and  the  watch- 
work  can  be  instantly  stopped  by  the  key. 

The  magnesium  light  is  surpassed  in  strength  by 
Drummond's  lime  light.  This  is  produced  by  a  gas  or 
spirit  flame,  into  which  oxygen  gas  is  blown.  The 
oxygen  gas  is  produced  by  a  salt  rich  in  that  element, 

*  1  groschen  =  six-fifths  of  a  penny,  or  I^d.  nearly.  1  gramme  =  the 
1000th  part  of  a  cubic  metre,  about  nine  solid  feet  of  water  at  the 
ordinary  average  temperature. 


CHEMICAL  EFFECT  OF  DIFFERENT  SOURCES  OF  LIGHT.      71 

the  chlorate  of  potassium.  This  salt  contains  the  oxygen 
combined  with  a  solid.  Being  heated,  it  escapes  as  a  gas, 
and  is  received  into  an  india-rubber  bag.  (See  Fig.  19.) 

This  bag  is  closed  by  means  of  a  stopcock,  and  when 
used  is  placed  between  two  pieces  of  wood  b  b,  a  weight 
being  placed  on  the  upper  one.  By  the  pressure  of  this 
weight  the  oxygen  gas  pours  through  the  cock  h,  and 
the  india-rubber  pipe  n,  into  the  oxygen  lamp  D.  To 
this  is  attached  a  burner  H  F,  running  into  the  point 
I.  The  lighting  gas  which  serves  for  combustion  enters 
through  the  cock  L,  which  is  connected  with  a  gas 
tube. 

The  combustion  takes  place  at  the  point  J.  Without 
oxygen  the  lighting  gas  burns  with  a  clear  but  soot- 
producing  flame ;  but  as  soon  as  the  oxygen  is  turned 
on,  the  flame  becomes  smaller  and  blue  in 
colour,  and  burns  with  an  intense  heat. 

Its  illuminating  power  is  small,  but  as 
soon  as  the  flame  has  brought  the  lime 
cylinder  to  a  red  heat,  a  dazzling  white  light 
issues  from  it,  which  has  a  very  intense 
effect  in  photography,  and  has  been  used 
with  success  by  Monckhoven  and  Harnecker 
to  produce  pictures  on  an  enlarged  scale. 

The  same  apparatus  serves  for  the  production  of  what 
are  called  cloud  pictures. 

The  electric  light,  produced  by  help  of  an  electric 
battery,  has  a  still  more  powerful  effect  than  the  lime 
light. 

If  a  piece  of  cannel  coal  (k  Fig.  20)  and  a  piece  of  zinc 
are  dipped  together  into  an  acid  (diluted  nitric  acid  or 
sulphuric  acid),  electricity  is  developed,  wrhich  produces 


72 


THE    CHEMISTRY    OF    LIGHT. 


a  spark  on  bringing  together  the  two  ends  of  the  zinc 
arid  coal  rising  above  the  fluid ;  this  spark  is,  however, 
very  feeble.  But  if  several  vessels  containing  zinc 
cylinders  z  and  pieces  of  charcoal  k  are  employed,  the 

spark  becomes  very  intense; 
and,  as  we  are  able  to  increase 
to  any  extent  the  number  of 
these  elements,  we  are  able  to 
produce  a  cone  of  light  of  any 
degree  of  brilliancy,  exceeding 
all  other  artificial  light. 

In    arranging    electric    bat- 
teries of  this  land,  the  zinc  of 
one  element  is  connected  with 
the  charcoal  of  the  following, 
andlhe  zinc  of  the  latter  with 
the  charcoal   of  the  third  ele- 
ment.    (See  Fig.  22.) 
If  the  two  wires  issuing  from  Z  and  C  are  brought 
together,  a  spark  of  light  is  produced  by  the  electric 
stream,  burning  the  metal  wire. 


Fig.  21. 


Fig.  22. 

The  light  is   generally  produced  between    cones   of 
charcoal  placed  in  front  of  a  concave  mirror  (Fig.  23). 


CHEMICAL  EFFECT  OF  DIFFERENT  SOURCES  OF  LIGHT.       73 

The  apparatus  S  and  Sf  serve  to  approach  or  withdraw 
the  cones,  while  the  upper  one  is  connected  with  the 
wire  K  by  the  foot  F ;  the  lower  one  is  connected  with 
the  wire  Z  of  the  electric  battery.  Thirty-six  elements 


Fig.  23. 

similar  to  those  of  Fig.  21  suffice  to  produce  the  electric 
light. 

The  arrangement  of  the  battery  makes  the  application 


74 


THE   CHEMISTRY   OF   LIGHT. 


of  the  light  inconvenient.     In  other  respects  this  light 
surpasses  all  others  in  photographic  effect. 

Nadar  has  made  with  it  many  excellent  pictures  in 
the  catacombs  of  Paris.  It  has  also  been  used  to  take 
portraits.  But  in  the  latter  case,  the  employment  of 
such  a  dazzling  artificial  light  is  attended  with  the 
drawback  of  occasioning  harshly  defined  shadows,  that 
disfigure  the  portrait. 

,  It  has  been  attempted  to  evade  this  by  allowing  an 
electric  light  of  less  power  to  operate  on  the  shaded  side ; 
but  it  is  difficult  under  this  dazzling  light,  as  in  the  sun- 
light, to  prevent  the  contraction  of  the  features. 


Fig.  24. 

It  thus  appears  that  all  these  artificial  lights  are  only 
auxiliaries  to  photographic  purposes,  especially  as  they 
are  so  expensive.  Accordingly,  their  use  will  be  confined 
to  places  that  cannot  be  lighted  in  any  other  way.  The 
writer  of  the  present  work  has  used  sunlight  with  great 
advantage  when  engaged  in  photographing  Egyptian 
sepulchres.  He  brought  the  light  into  subterranean 
passages  by  means  of  reflection. 


CHEMICAL  EFFECT  OF  DIFFERENT  SOURCES  OF  LIGHT.      75 

Let  the  reader  imagine  a  mirror  set  up  in  the  open 
air,  reflecting  the  sun's  rays  through  the  sepulchral 
entrance  T,  into  the  subterranean  vault  G.  In  this 
vault  they  are  received  by  a  second  mirror,  which  throws 
the  rays  on  the  surface  of  wall  W,  of  which  a  photo- 
graph has  to  be  taken.  I  admit  that  nothing  but  a  speck 
of  light  is  thus  received ;  but  if,  during  the  time  of 
exposing  the  photographic  plate,  this  speck  be  allowed  to 
move  over  the  part  of  wall  W,  of  which  a  photograph 
is  to  be  taken,  all  parts  of  the  object  receive  successively 
enough  light  to  allow  of  photographic  effects.  The 
movement  of  the  speck  over  the  wall  is  effected  by  the 
agitation  X)f  mirror  b. 

Braun  of  Dornach,  by  help  of  the  same  procedure,  was 
able  at  a  later  date  to  reproduce  the  very  dark  frescoes 
of  Eaphael  and  Michael  Angelo  in  the  Sixtine  Chapel 
and  in  the  galleries  of  the  Vatican,  and  produced  ex- 
cellent results. 

The  sunlight  remains  the  most  important  source  of 
light  for  photographic  purposes.  The  clearness  of  this 
light  is;  however,  exposed  to  great  variations.  Even  the 
naked  eye  recognizes  that  the  sun  is  much  brighter  at 
noon  than  in  the  morning  and  evening.  According  to 
the  measurements  of  Bouguer,  this  difference  is  so  con- 
siderable, that  the  sun  at  an  elevation  of  50S  above  the 
horizon  is  1200  times  brighter  than  at  sunrise.  The 
eye,  moreover,  perceives  a  decided  difference  of  colour 
between  the  sun  on  the  horizon  and  the  sun  at  the 
zenith.  The  latter  appears  white,  th£  former  of  a 
more  reddish  hue  ;  and,  on  making  experiments  with 
the  spectrum  apparatus,  it  is  found  that  in  the  setting 
sun  the  reddish  rays  predominate,  while  the  blue  and 
violet  are  in  part  wanting. 


76 


THE   CHEMISTKY   OF   LIGHT. 


It  follows  hence,  that  the  chemical  effect  of  the  sun- 
light is  very  feeble  in  the  morning  and  the  evening ;  that 
it  increases  as  the  sun-rises  above  the  horizon,  and  that 
it  attains  its  greatest  intensity  about  noon. 

The  cause  of  the  red  hue  of  the  morning  and  evening 
sun  is  found  in  the  fact  that  the  particles  of  air  partly 
repel  the  blue  rays — for  which  reason  the  air  (that  is, 
the  sky)  appears 'blue — whereas  they  admit  the  yellow 
and  red  rays  more  easily. 

If  E  (Fig.  25)  is  the  earth  surrounded  by  the  atmo- 
sphere A,  S  the  sun  at  moment  of  sunrise,  S"  the  sun 
at  the  moment  of  sunset  for  the  place  0,  and  S '  the 
sun  at  noon,  it  is  apparent  that  the  sun's  rays  at  sunrise 
and  sunset, .have,  to  .travel  much  farther — namely,  the 
distance  betwcdh  Vl  and  0 — than  when  the  sun  is  at  the 


Fig.  25. 

zenith  S'.  But  in  proportion  as  the  stratum  of  atmo- 
sphere through  which  the  sun  must  pass  to  arrive  at  the 
spectator,  the  weaker  it  becomes.  It  follows  from  this 
that  on  high  mountains  the  chemical  effect  of  the  rays 
of  light  must  be  more  intense,  and  this  has  been  proved 
by  experiments  on  the  Alps. 


CHEMICAL  EFFECT  OF  DIFFERENT  SOURCES  OF  LIGHT.      77 

But  not  only  are  chemical  effects  produced  by  the 
sunlight ;  the  blue  sky,  which  is  nothing  but  reflected 
sunlight,  is  likewise  operative,  and  powerfully  so,  through 
its  blue  colour. 

It  has  been  already  stated  that  the  blue  colour  of  the 
sky  proceeds  from  this,  that  the  particles  of  the  air 
reflect  more  especially  blue  light.  But  the  quantity  of 
this  reflected  blue  light  varies  with  the  hour  of  the  day, 
being  strongest  when  the  sun  is  highest  (that  is,  at 
noon),  and  it  diminishes  in  proportion  as  the  sun 
approaches  the  horizon.  Therefore  photographers  are 
wont  to  take  their  photographs  of  portraits  when  they 
only  use  the  light  of  the  blue  sky,  at  noon;  that  is, 
between  10  a.m.  and  2  p.m.  During  these  hours  the 
chemical  effect  of  light  remains  almost  the  same ;  after- 
wards it  diminishes  rapidly, — quicker  in  winter,  slower 
in  summer.  Thus  the  chemical  power  of  light,  accord- 
ing to  Bunseii,  expressed  in  degrees,  is  at  Berlin : — 

1 2  o'clock.  1  o'clock.  2  o'clock.  3  o'clock.  4  o'clock.  5  o'clock.  6  o'clock.  7  o'clock.  8  o'clock. 
From  June  21  38°  38  38  37  35  30  24  14  6 

From  Dec.  21  20°  18  15  9  0 

It  appears  from  this  example  how  extraordinarily 
weak  is  chemical  light  in  winter  (for  example,  towards 
noon  on  the  21st  December  about  half  as  powerful  as 
towards  noon  on  the  21st  June) ;  moreover,  how  small 
the  amount  of  chemical  light  is  which  is  diffused  by  the 
blue  sky  on  the  21st  December,  on  account  of  the  short- 
ness of  the  day.  Therefore  photographers  must  expose 
much  longer  in  winter  than  in  summer,  and,  their 
printing  process  being  slower,  they  take  much  longer  in 
winter  to  copy  the  same  number  of  pictures. 


78  THE   CHEMISTRY   OF  LIGHT. 

Now  th$  intensity  of  tie  blue  sky  light  depends  on 
the  position  of  the  sun,  and  the  latter  varies,  not  only 
according  to  the  different  seasons,  but  also  at  the  very 
same  seasons  on  different  parts  of  the  earth. 

If  circles  be  drawn  round  the  earth  from  pole  to  pole, 
we  obtain  what  are  called  meridians  (m  m  Fig.  26).  All 
places  that  are  situated  on  the  same  meridian  have 
noon  at  the  same  time,  but  the  height  of  the  sun  varies 
very  much  according  to  the  distance  of  the  place  from 
the  equator. 

If  circles  be  drawn  round  the  earth  parallel  to  the 
equator,  they  form  what  is  called 
lines  of  latitude.  If  the  sun  is  at  a 
particular  place  on  the  equator  per- 
pendicular at  noon,  at  the  10°  of 
north  latitude  it  is  10°  lower;  that 
is,  the  height  of  the  sun  (or  the  dis- 
tance of  the  sun  from  the  horizon 
Flg<  2  '  expressed  in  angular  measurement) 

is  80°.  At  10°  further  north,  the  position  of  the  sun  at 
the  same  time  is  only  70°;  and  at  the  pole,  which  is 
90°  from  the  equator,  the  height  of  the  sun  =  0 ;  that 
is,  the  sun  is  on  the  horizon. 

The  chemical  ^strength  of  the  blue  sky  light  varies 
greatly,  corresponding  to  the  different  positions  of  the 
sun  at  the  same  time.  Thus,  for  example — 

At  Cairo,  on  the  21st  Sept.,  the  strength  of  light  at  noon  =  105° 
At  Heidelberg  „  „  „  =    57° 

At  Iceland  n  „  „  =    27° 

Therefore,  the  more  southerly  a  place  is,  the  richer  it 
is  in  the  amount  of  light  it  offers  to  the  photographer. 


CHEMICAL  EFFECT  OF  DIFFERENT  SOURCES  OF  LIGHT.       79 

Accordingly,  the  American  photographers  are  better  off 
than  those  of  Germany  and  England. 

These  differences  in  the  intensity  of  chemical  light  are 
yet  essentially  modified  by  the  state  of  the  weather.  If  the 
sky  is  covered  with  grey  clouds,  the  chemical  intensity 
of  light  is  considerably  less  than  with  a  perfectly  clear 
sky.  On  the  other  hand,  white  clouds  increase  the  chemi- 
cal intensity  of  light  very  decidedly.  In  the  autumn  the 
chemical  intensity  of  light  is  much  greater  than  in  spring, 
perhaps  in  consequence  of  the  greater  transparency  of 
the  air.  According  to  Eoscoe,  it  is  in  August  and  Sep- 
tember more  than  one  and  a  half  times  as  great. 

These  variations  in  the  chemical  intensity  of  light  are 
very  important  to  the  life  of  plants.  The  green  leaves 
of  plants  inhale  carbonic  acid  and  exhale  oxygen  under 
the  influence  of  light.  But  this  breathing  process  does 
not  take  place  without  the  presence  of  light.  The  green 
colour  of  leaves  and  the  variegated  scale  of  colours  in 
flowers  only  exist  under  the  operation  of  light.  In  the 
dark,  plants  only  develop  sickly  blossoms,  like  the  well- 
known  white  germs  of  potatoes  kept  in  cellars. 

The  necessity  of  light  for  the  life  of  plants  is  also  seen 
in  the  effort  made  by  plants  kept  in  darkened  rooms  to 
reach  the  apertures  which  admit  light,  growing  as  it 
were  towards  them.  Hence  a  plant  develops  with  an 
energy  proportioned  to  the  intensity  of  the  light.  Ac- 
cordingly, the  greater  fruitfulness  of  the  tropics  is  to  be 
ascribed,  not  only  to  the  higher  temperature,  but  also  to 
the  greater  chemical  intensity  of  light.  Eeeent  observa- 
tions have  established  that  the  yellow  and  red  rays,  and 
not  the  blue  and  violet,  produce  the  greatest  chemical 
effect  on  the  leaves  of  plants. 


80  THE    CHEMISTRY   OF   LIGHT. 

We  have  now  arrived  at  the  knowledge  of  the  import- 
ance of  light  for  the  economy  of  nature.  The  atmo- 
spheric air  consists  of  two  kinds  of  gas,  oxygen  and 
nitrogen,  which  are  in  combination.  Nitrogen  is  a 
perfectly  innocuous  kind  of  air,  serving  to  attenuate  the 
oxygen;  for  the  latter  alone,  though  essential  to  life, 
would  be  injurious. 

In  breathing,  part  of  the  oxygen  is  absorbed  in  the 
lungs:  it  forms,  with  the  organic  comtituent  parts  of  the 
body,  carbonic  acid  and  water.  The  carbonic  acid  and 
water  are  exhaled  by  us  and  dispersed  again  in  the  air. 

It  is  easy  to  prove  by  an  experiment  that  a  consider- 
able amount  of  carbonic  acid  is  contained  in  the  air  we 
exhale.  Carbonic  acid  forms,  combined  with  lime-water, 
an  insoluble  precipitate  called  carbonate  of  lime.  If  now 
we  exhale  through  a  glass  tube,  letting  our  breath  pass 
into  the  perfectly  clear  lime-water,  the  latter  becomes 
troubled  by  the  formation  of  carbonate  of  lime.  Hence 
the  amount  of  oxygen  in  the  atmospheric  air  is  con- 
tinually diminished  and  converted  into  carbonic  acid. 
The  same  result  is  produced  on  a  larger  scale  by  the 
process  of  combustion.  In  this  process  a  combination 
takes  place  of  wood  or  coal  with  oxygen,  and  the  result  is 
again  principally  carbonic  acid. 

It  might  be  supposed  from  this  that,  in  the  course  of 
time,  the  amount  of  oxygen  in  the  air  must  diminish, 
while  that  of  carbonic  acid  would  increase.  This 
actually  takes  place  in  closed  spaces.  Leblanc  found 
that,  after  a  lecture  in  one  of  the  lecture  rooms  of  the 
Sorbonne  at  Paris,  the  air  had  lost  one  per  cent,  of 
its  oxygen. 

In  the  open  air  this  diminution  of  oxygen  and  increase 


CHEMICAL  EFFECT  OF  DIFFEREN1 

of  carbonic  acid  gas  is  not  noticed,  and  the  reason  of  this 
is  that  the  carbonic  acid  formed  by  combustion  and  the 
.exhalations  of  animals  is  again  decomposed  by  plants 
under  the  influence  of  light. 

Plants  absorb  the  carbonic  acid,  retaining  the  carbon 
I  and  liberating  the  oxygen;  by  which  means  the  latter,  lost 
by  combustion  and  exhalation,  is  made  again  available. 

There  was  a  time  when  the   atmosphere  was   much 
richer   in    carbonic   acid    gas    than    now.     When    the 
!  incandescent  and  fluid   masses  that  once   formed  our 
;  earth   gradually  became   condensed,  when  the  watery 
vapours  were  precipitated  as  seas,  the  atmosphere  con- 
tained almost  all  the  carbon  of  the  earth  after  combus- 
tion ;  that  is,  united  with  oxygen  as  carbonic  acid  gas. 
The  air  was  therefore  at  that  time  infinitely  richer  in 
carbonic  acid  than  now.     When  at  length  the  earth  had 
cooled  sufficiently  for  vegetation  to  be  developed,  gigantic 
plants    shot   forth  from    the  warm   ground   under  the 
;  influence  of  the  sunlight.    They  flourished  luxuriantly  in 
I  the  atmosphere  rich  in  carbonic'  acid,  the  carbon  of  the 
carbonic  acid  passed  over  into  the  form  of  wood,  and  thus 
in  the  course  of  thousands  of  years  it  was  continuously 
diminished.      Eevolutions   of  the   earth's   surface  suc- 
ceeded ;   whole  territories  with  their  forests  were  buried 
under  sand  and  clay  beds,  and,  becoming  decomposed, 
were  changed  into  coal/     A  fresh  vegetation  sprouted 
forth  from  the  newly-formed  soil,  and  again  absorbed, 
under  the  influence  of  light,  the  carbonic  acid  of  the 
atmosphere,  to  be  once  more  engulphed  by  a  fresh  cata- 
clysm.    Thus,  the  carbon  from  the  carbonic  acid  of  the 
atmosphere  was  stored  as  coal  in  the  depths  of  the  earth ; 
and  thus  the  atmosphere,  by  the  chemical  effect  of  light, 


82  THE    CHEMISTRY   OF   LIGHT. 

became  continually  richer  in  oxygen,  until  at  length, 
after  countless  revolutions  of  the  earth,  it  obtained  that 
wealth  of  oxygen  which  made  the  existence  of  man 
possible,  when  he  appeared  at  the  end  of  the  earth's 
development. 

We  see,  therefore,  that  the  chemical  influence  of 
light  has  played  an  important  part  in  the  development 
of  our  planet,  and  it  continues  to  do  so  in  the  economy 
of  nature. 


CHAPTEE  IX. 

ON  THE  REFRACTION  OF  LIGHT. 

Simple  Refraction— Deviation — Index  of  Refraction — Refraction  in  Glass 
Plates — Prisms  and  Lenses — Production  of  Prisms  or  Images  by 
Lenses. 

WE  have  already  pointed  out  (p.  60)  that  when  a  ray 
of  light  passes  the  border  of  two  transparent  media  of 
unequal  density,  a  change  of  direction 
takes  place  which  is  called  refraction. 

If  a  small  coin  is  placed  in  an  opaque 
vessel,  and  the  eye  0  be  kept  in  such  a 
position  that  the  edge  of  the  vessel 
covers  the  coin,  it  is  invisible.  But  if 
water  be  poured  into  the  vessel  the  coin  Fig.  27. 

becomes  visible,  and  this  takes  place  by  the  refraction 
which  the  rays  experience  in  passing  from  water  to  air. 
(See  Fig.  27.) 

The  angle  which  the  united  rays  make,  before  and 
after  the  refraction,  is  called  the  deviation. 

This  deviation  increases  in  proportion  to  the  oblique- 
ness with  which  the  rays  fall  upon  the  surface  of  the 
water. 

In  order  to  determine  exactly  the  degree  of  the 
refraction,  let  a  perpendicular  line  be  conceived  to  be 


THE    CHEMISTRY   OF    LIGHT. 


Fig.  28. 


erected  at  the  point  of  immersion  n  of  the  ray  n  I  (Fig. 
28).  This  line  is  called  the  normal,  or  plumb  line,  and 
the  angle  i  which  the  ray  forms 
with  this  normal  is  called  the 
angle  of  incidence,  while  the  angle 
r  which  the  refraction  ray  forms 
with  the  same  normal  is  called  the 
angle  of  refraction. 

The  ratio  of  the  magnitude  of 
the  angle  of  incidence  to  the  angle 
of  refraction  is  peculiar.  If  a  circle 
be  described,  and  from  points  a 
and  b  perpendicular  lines  a  d  and  &/are  let  fall  on  the 
normal,  the  result  obtained  is  what  mathematicians  call 
the  sine  of  an  angle.  Thus  a  d  is  the  sine  of  i,  and  bj 
the  sine  of  r.  The  ratio  of  the  sine  of  incidence  to  the 
sine  of  refraction  is  constant. 

This  ratio  is  when  light  leaves  air  for  water  as  4  to  3 ; 
that  is,  the  sine  bfis.  f  times  as  great  as  sine  a  d,  or  sine 
a  d  is  J  times  greater  than  sine  bf.     Light  is  still  more 
refracted  on  entering  glass.     In  this 
case  the  ratio  of  the  sines  is  as  3  to 
2.     This  ratio  of  the  sines  of  the  two 
angles  is  designated  by  the  name  ex- 
ponent of  refraction,  or  index  of  re- 
fraction. 

If  a  ray  of  light  n  I  falls  upon  a 
smooth  sheet  of  glass,  it  experiences 
a  similar  refraction;  it  continues  in  the  direction  n  n, 
and  the  angle  of  refraction  at  n  on  the  glass  becomes 
two-thirds  of  the  angle  of  incidence.  (Fig.  29.) 

On  issuing  from  the  other  side  of  the  sheet  of  glass, 
another  refraction  takes  place ;  but  in  this  case  the  angle 


Fig.  29. 


ON    THE    REFRACTION    OF    LIGHT. 


85 


of  refraction  at  ri  in  the  air  becomes  one  and  a  half 
times  larger  than  the  angle  on  the  glass,  and  as  the 
angle  at  n  is  equal  to  the  angle  at  ri,  the  angle  of 
emergence  r  nf  is  of  the  same  magnitude  as  the  angle 
of  immersion  n  I;  that  is,  the  ray  continues,  after 
refraction,  in  its  original  direction.  At  all  events,  it 
only  experiences  a  prolongation  parallel  with  itself. 
Therefore  we  see  through  our  windows  in  the  same 
direction  in  which  they  are  really  situated. 

The  ratio  is  entirely  different 
when  the  spectator  looks  through 
a  glass  having  three  faces.  If  the 
eye  is  at  o,  and  an  object  at  a,  and 
a  prism  with  three  faces  be  held 
close  to  the  eye,  the  object  is  not 
seen  at  a,  but  in  the  direction  of  a'.  The  incident  ray 
a  d  suffers  a  deviation  at  the  first  face  of  the  glass, 
taking  the  direction  d  c ;  at  the  refraction  on  the  second 
face  it  makes  another,  o  c.  Both  deviations  correspond. 


Pi".  30. 


Fig.  31. 

• 

The  greater  the  magnitude  of  the  angle  x  which  the 
two  faces  of  the  prism,  through  which  the  ray  passes, 
make  with  each  other,  the  greater  is  this  deviation. 


86 


THE    CHEMISTKY   OF   LIGHT. 


Thus  the  deviation  at  prism  d  is  greater  than  at  prism 
c,  and  at  prism  a  it  is  greater  than  at  prism  b ;  because 
the  angle  of  refraction  x  in  b  is  greater  than  in  c,  and  at 
a  it  is  greater  than  in  b. 

If  a  glass  structure  be  erected,  consisting  of  separate 
prisms  of  different  angles,  and  if  a  bundle  of  parallel 
rays  be  conceived  to  fall  upon  it,  the  ray  a  is  more 


^t>- 
c 


Fig.  32. 

strongly  refracted  than  the  ray  b  falling  on  the  more 
pointed  prism,  and  the  latter,  again,  is  more  refracted 
than  ray  c  falling  on  the  still  more  pointed  prism,  and 
the  result  is  that  all  the  rays  unite  in  one  point  /. 

If  instead  of  the  prisms 
we  substitute  a  connected 
symmetrical  mass  of  glass, 
we  obtain  the  section  of  a 
burning  glass,  or,  as  the 
opticians  say,  a  lens,  which 

has  the  property,  as  in  the  illustration,  of  uniting  all 
parallel  incident  rays  in  one  point.  (See  Fig.  33.) 


ON    THE    REFRACTION   OF   LIGHT. 


87 


Every  lens  is  contained  between  two  spherical  faces. 
The  connecting  line  running  through  the  centre  of  both 
spherical  surfaces  is  named  the  axis  of  the  lens,  and 
point  E  (Fig.  33),  where  the  parallel  incident  rays 
unite,  is  the  focus,  while  its  distance  from  the  lens  is 
the  focal  distance.  But  not  only  are  the  parallel 
incident  rays  united  in  one  point  by  the  refraction  in  a 
lens  of  this  kind, — the  same  thing  occurs  with  all  rays 
issuing  from  the  same  point.  Their  converging  point  is 
named  principal  focus. 


'  Fig.  34. 

A  luminous  point  S,  for  example,  directs  a  cone  of  rays 
to  the  lens.  After  refraction  these  are  united  at  E.  If 
S  be  brought  nearer  to  the  lens,  R  removes  further ;  if 
S  be  brought  so  near  that  its  distance  from  the  lens 
is  twice  the  focal  distance,  then  the  converging  point  B 
is  equally  distant  from  the  lens. 


Fig.  35. 


If  instead  of  the  luminous  point,  an  object,  for  example 
an  arrow  B  A,  is  placed  before  the  lens,  each  individual 
point  of  the  object  sends  out  a  cone  of  light  to  the  lens, 


88  THE   CHEMISTRY   OF   LIGHT. 

and  all  the  rays  of  one  and  the  same  cone  converge  to 
one  point,  the  rays  issuing  from  A  to  a,  and  those 
issuing  from  B  to  I;  and  the  result  is  that  a  perfect 
miniature  and  inverted  image  of  the  arrow  is  produced. 

If  the  arrow  be  moved  nearer  to  the  lens,  its  picture 
is  removed  farther  from  the  lens  and  is  magnified.  For 
example,  if  the  little  arrow  a  &  is  placed  before  the  lens, 
it  produces  the  enlarged  picture  B  A. 

But  if  the  arrow  be  removed  farther  from  the  lens,  its 
image  approaches  continually  nearer  to  the  lens,  and 
therefore  becomes  continually  smaller.  Accordingly,  a 
lens  is  able  to  project  enlarged  or  diminished  images  of 
an  object,  by  the  latter  being  approached  to  or  removed 
from  it. 


CHAPTEE  X. 

THE  PHOTOGRAPHIC  OPTICAL  APPARATUS. 

Construction  of  the  Camera  Obscnra — Telescopic  Images — The  Magic 
Lantern — Magnifying  Apparatus — The  Stereoscope. 

WE  have  just  shown  that  a  lens  is  able  to  produce 
enlarged  and  diminished  pictures  of  objects  according  to 
their  distance.  On  this  principle  depends  the  working  of 


Fig.  36. 

the  camera  obscura,  the  most  important  photographic 
apparatus,  which  serves  to  project  plane  pictures  of  solid 
objects  in  nature.  We  have  already  described  (see  p.  7) 
its  simplest  form.  It  is  a  dark  chamber  having  a  small 
hole  in  its  lid.  This  arrangement  produces  very  indistinct 


90  THE    CHEMISTRY   OF   LIGHT. 

pictures  deficient  in  light.  But  if  a  lens  is  placed  in  the 
dark  slide  o  (Fig.  36),  this  produces  on  the  opposite 
wall  a  picture  of  the  objects  facing  the  chamber,  which  is 
much  clearer  and  more  sharply  defined  than  the  image 
produced  through  the  aperture.  It  is  evident  that  in 
this  case  the  distance  of  the  wall  must  correspond  to  the 
distance  of  the  image.  Now,  as  this  varies,  in  order  to 
determine  the  place  where  the  image  is  found  the 
camera  has  been  converted  into  a  small  dark  box  (Fig. 
37),  the  back  part  of  which  is  movable,  and  contains  the 

ground  glass  slide  g. 
If  the  back  of  the 
camera  o  is  moved  to 
and  fro,  it  is  easy  to 
find  the  situation  of  an 
image  of  an  object 
placed  before  the  lens  Z. 
In  order  to  ascertain 
this  distance  with  the 
necessary  accuracy,  photographic  lenses  have  been 
supplied  with  a  screw  having  a  spring  r  at  the  frame 
of  the  lens ;  but  this  addition  is  by  no  means  necessary. 
In  order  to  be  able  to  see  on  the  ground  glass  slide  g, 
all  foreign  light  that  blinds  the  eye  must  be  kept  off,  and 
to  this  end  a  dark  cloth  is  thrown  over  the  head,  forming 
what  is  called  the  black  curtain. 

The  operation  of  seeking  the  image  is  named  in 
photography  focussing.  It  follows  from  what  has  been 
said  that  the  image  appears  inverted  on  the  ground 
glass  slide.  Though  the  process  of  seeking  the  image 
appears  simple  at  first  sight,  it  is  really  rendered 
difficult  by  the  fact  that  objects  at  different  distances 


THE   PHOTOGRAPHIC   OPTICAL  APPARATUS. 


91 


present  images  that  likewise  vary  in  dis- 
tance from  the  ground  glass  slide.  For 
example,  if  a  head  be  placed  opposite  a 
camera,  the  nose  is  nearer  to  the  lens 
than  the  hairs  behind  the  head ;  and  the 
result  is  that  the  image  of  the  nose  in  ; 
the  camera  is  farther  from  the  lens  than 
the  hairs  of  the  back  or  side  of  the  head. 
Accordingly,  the  whole  head  never  pre- 
sents the  same  degree  of  sharpness  or 
definite  accuracy.  Photographers  are 
satisfied  with  rendering  the  main  points 
with  definite  clearness,  such  as  the  face, 
and  they  bestow  less  care  on  the  subordi- 
nate parts. 

If  the  situation  of  the  object  be  rather 
remote  (for  example,  a  landscape,  whose 
nearest  features  in  the  foreground  are 
about  fifty  times  as  remote  as  the  focus), 
the  images  of  the  different  features,  what- 
ever their  remoteness  may  be,  appear 
all  in  the  same  focus. 

The  same  thing  occurs  in  the  case  of 
stars.  Photographic  cameras  are  well 
adapted  to  project  images  of  stars,  only 
they  are  very  small  if  the  focus  of  the 
lens  is  smalL  Accordingly,  telescopic 
lenses  are  preferred  in  such  cases.  The 
production  of  images  in  these  cases  rests 
on  the  very  same  principles  as  the  pro-' 
duction  of  images  in  other  lenses.  If  we 
imagine  the  telescopic  lens  o  o,  and  place 


92  THE    CHEMISTRY    OF    LIGHT. 

before  it  at  a  great  distance  the  arrow  A  B,  it  produces 
a  very  diminished  image  of  the  arrow  I  a.  Thus,  the 
image  of  the  sun  at  a  distance  of  90  millions  of  miles  is, 
in  the  focus  of  a  lens  six  feet  wide,  only  eight  lines  in 
size.  If  it  is  wished  to  obtain  the  photograph  of  such 
an  image,  the  tube  R,  on  which  the  lens  L  is  fixed, 
must  be  arranged  as  a  photographic  camera.  (See 
Fig.  39.)  A  movable  ground  glass  shade  n  is  brought 
up  behind,  to  throw  out  a  sharply-defined  image,  and 
this  can  be  exchanged  for  a  photographic  plate  during 
the  exposure.  This  is  the  method  adopted  by  Warren 
de  la  Eue,  Eutherford,  and  others  engaged  in  expeditions 
to  determine  eclipses,  and  also  by  the  author  in  the 
expedition  to  Aden  in  1868. 


Fig.  39. 

The.  images  taken  by  a  photographer  are  usually 
smaller  than  nature.  But  he  is  also  in  a  position  to  pro- 
duce images  that  are  larger  than  the  originals.  Every 
lens  gives,  as  described  at  p.  88,  different  images  of  the 
same  object,  varying  according  to  distance.  If  the 
object  is  nearer  than  twice  the  focal  distance,  an 
enlarged  image  is  produced,  but  if  it  is  more  remote,  the 
image  is  smaller.  The  latter  is  the  commoner  case. 
Nevertheless,  to  make  enlarged  images  direct  from  nature 
is  attended  with  difficulties.  The  larger  the  image,  the 
larger  is  the  surface  over  which  the  light  is  dispersed 
which  issues  from  the  objects,  and,  accordingly,  the 


THE    PHOTOGRAPHIC    OPTICAL   APPARATUS. 


93 


smaller  will  be  the  amount  of  light  over  each  p  art  of  the 
image.  But,  in  proportion  as  an  image  is  deficient  in 
light,  the  larger  must  be  the  exposure  to  produce  a 
photographic  impression.  A  man  would  find  it  difficult 
to  endure  such  a  long  sitting,  therefore  this  method  is 
only  employed  in  drawings  and  the  like. 

Enlarged  pictures  of  other  objects  are  represented  by 
the  help  of  an  apparatus  resembling  the  magic  lantern. 
The  magic  lantern  consists  in  the  production  of  an 
enlarged  image  by  means  of  lenses.  Instead  of  a  simple 
lens,  a  system  of  lenses  n  n  o  o  is  employed  for  enlarging, 
and  gives  more  sharply  defined  images.  The  image  that 
is  painted  or  photo- 
graphed on  glass  is 
slid  in  by  a  lateral 
slide  and  brightly  illu- 
mined. To  this  end 
a  lamp  L,  the  concave 
mirror  H,  and  the  lens 
m  m  are  employed. 
These  concentrate  the 
clear  lamplight  on  the 
image  that  has  to  be 
enlarged.  According  as 
lens  n  o  is  approached 
or  withdrawn,  —  that 
is,  according  to  the  variation  of  its  distance  from  its 
original,— the  images  obtained  are  larger  or  smaller  in 
size. 

This  instrument  was  formerly  nothing  but  a  plaything, 
but  it  has  become  latterly  an  important  auxiliary  in 
instruction.  Photographs  of  microscopic  preparations, 


94  THE    CHEMISTRY   OF   LIGHT. 

of  animals,  plants,  minerals,  landscapes,  national  types 
and  architecture,  give  in  this  manner  a  more  faithful 
and  a  truer  representation  than  the  maps  and  tables, 
which  are  in  general  very  imperfectly  designed. 

In  America  this  application  of  the  magic  lantern  is 
universal.  Every  considerable  educational  establish- 
ment possesses  one  such  instrument,  and  often  more.  In 
Germany,  though  so  useful,  it  has  been  hitherto  left  in 
the  hands  of  the  traders  connected  with  the  annual 
fairs,  who  employ  them  for  what  are  called  cloud 
pictures.  These  cloud  pictures  are  produced  by  the 
help  of  two  magic  lanterns  placed  side  by  side,  both  of 
which  project  their  images  on  the  same  screen.  If  one 
of  these  lenses  be  covered  up,  one  of  the  pictures  dis- 
appears and  the  other  alone  remains  visible.  Mean- 
while, if  another  image  be  substituted  for  the  one  with- 
drawn, and  the  lid  again  taken  off,  a  combination  of  two 
images  is  obtained.  If  the  lens  be  only  gradually,  not 
suddenly,  closed,  the  image  also  fades  away  gradually, 
until  it  disappears  entirely. 

Professor  Czarmak,  at  Leipsic,  has  latterly  introduced 
the  representation  of  enlarged  images  by  the  magic 
lantern  as  an  important  auxiliary  in  his  lectures ;  and 
he  obtained  so  great  a  success  with  it  that  he  prepared 
the  way  for  its  general  introduction  into  schools. 

We  take  this  occasion  to  remark  that  wronderful  paint- 
ings on  glass  have  been  lately  produced  from  photo- 
graphs by  means  of  a  new  light -printing  process. 
These  glass  images  have  been  exposed  for  sale,  and  are 
specially  destined  for  the  magic  lantern.  Their  price  is 
so  moderate,  and  the  objects  they  represent  are  so  in- 
teresting—being landscapes  of  all  parts  of  the  earth— 


THE    PHOTOGRAPHIC    OPTICAL    APPARATUS. 


95 


that  it  is  within  the  reach  of  every  family  to  obtain  a 
collection  of  the  most  pleasing  and  entertaining  views. 
At  domestic  entertainments  by  the  family  fireside, 
images  of  this  kind  united  with  the  magic  lantern 
become  an  important  means  of  instruction  and  enjoy- 
ment to  young  and  old. 

A  petroleum  lamp  is  not  sufficient  for  the  representa- 
tion of  such  images  on  a  large  scale.  For  this  purpose 
more  powerful  applications  of  light  must  be  employed, 


Fig.  41. 

either  Drummond's  lime  light  or  the  electric  light  (see 
p.  72).  To  retain  such  enlarged  images  in  a  photo- 
graphic form,  a  sheet  of  sensitized  paper  is  stretched 
over  the  place  and  instead  of  the  screen. 

To  produce  photographic  images  life  size,  the  magic 
lantern  is  not  used,  but  the  solar  camera,  a  section  of 
which  is  represented  Fig.  41,  and  a  front  view  of  it 
Fig.  42. 


96  THE    CHEMISTRY   OF   LIGHT. 

Sunlight  is  suffered  to  fall  on  a  great  lens  B,  which 
0  oncentrates  it  on  a  small  negative     N9  and  close  to  it  is 
the  objective  O,  which  projects  an  enlarged  image  on  the 
screen  E.     The  image  is  evidently  negative.     If  a  sensi- 
tized piece  of  paper  be  stretched  at  R,  this  paper  be- 


Fig.  42. 


comes  brown  at  all  places  where  the  negative  is  clear 
(transparent),  and  it  remains  white  in  all  places  where 
the  negative  is  black  (opaque);  therefore  the  result  is  a 
positive.  The  whole  apparatus  is  enclosed  in  a  darkened 
wooden  box,  which  can  be  shifted  by  means  of  cog 


C4 

PH 


THE   PHOTOGRAPHIC    OPTICAL    APPARATUS.  97 

wheels  and  a  winch,  so  that  it  can  always  be  turned 
towards  the  sun. 

In  conclusion,  we  have  to  describe  a  most  beautiful 
optic-photographic  apparatus,  which  permits  us  to  see 
images  not  only  as  plane  objects,  but  as  solid  bodies. 
This  is  the  stereoscope. 

Our  readers  know  already  that  this  instrument  is 
intended  to  exhibit  double  pictures,  the  two  halves  of 
which  on  the  first  look  seem  to  be  absolutely  alike,  and 
which  when  viewed  through  the  instrument  form  one 
picture,  which  appears  no  longer  plane,  but  solid. 

The  two  pictures  which  are  seemingly  alike  are,  in 
fact,  different.  If  we  look  at  one  of  the  cubes  with  the 
right  eye,  we  see  rather  more  of  the  right  side ;  if  we 
look  with  the  left  eye,  we  see  something  more  of  the 
left  side,  taking  for  granted  that  the  head  is  not  moved. 
The  pictures  of  the  right  and  of  the  left  eyes  combine 
with  each  other  and  give  the  solid  appearance  or  im- 
pression. 

If  we  close  one  eye,  the  bodily  impression  is  far 
weaker;  the  objects  appear  plane  or  flat.  This  may 
not  be  readily  credited,  because  men  do  not  often  seek 
an  explanation  of  what  they  see,  but  look  at  objects 
much  too  hastily.  But  it  can  be  easily  ascertained 
that  such  is  the  fact  if  a  bottle  be  placed  before  a  wall, 
or  in  front  of  an  upright  book,  and  if  we  then  look 
at  it.  With  two  eyes  open  we  readily  perceive  the 
distance  of  the  bottle  from  the  wall  or  book,  but  directly 
we  close  one  eye  the  bottle  and  the  book  appear  to  be 
almost  contiguous,  and  it  is  only  by  moving  the  head  on 
one  side  that  we  clearly  distinguish  the  distance  between 
them. 


93  THE    CHEMISTRY   OF    LIGHT. 

Accordingly,  the  use  of  both  eyes  is  necessary  to  have 
a  proper  perception  of  a  bodily  impression.  It  is  only 
in  this  manner  that  we  come  to  the  conviction  that 
space  has  not  only  height  and  breadth,  but  also  depth. 
One-eyed  persons  only  receive  this  impression  by  turn- 
ing the  head  on  one  side.  If  objects  are  very  remote, 
the  difference  between  the  views  which  the  right  eye  and 
the  left  eye  have  of  them  is  very  inconsiderable ;  arid, 
accordingly,  such  remote  objects  appear  flat  and  without 
solidity,  and  it  is  only  when  we  change  our  position  and 
observe  it  from  different  sides  that  we  become  acquainted 
with  its  solidity.  This  is  therefore  purely  an  affair  of 
experience.  Every  person  will  recognize  a  remote  house 
as  a  solid  object,  because  we  know  from  experience  that 
a  house  is  a  solid ;  but  that  we  actually  see  it  as  a  flat 
surface  is  proved  by  the  deception  produced  by  thea- 
trical decorations,  where  the  remote  background,  if 
properly  painted,  often  produces  an  extremely  natural 
effect  ;  but  we  perceive  this  background  to  be  flat 
directly  we  move  the  head  on  one  side.  When  this  is 
done,  a  solid  object  presents  a  different  appearance,  but 
a  flat  surface  remains  unchanged. 

Wheatstone  was  impressed  by  the  fact  that  the  solid 
impression  made  by  an  object  is  the  combination  of 
different  representations  of  it  given  by  the  right  and  the 
left  eye.  Accordingly,  he  tried  to  substitute  for  one 
object  a  picture  of  the  right  side  of  it  for  the  right  eye, 
and  a  picture  of  the  left  side  of  it  for  the  left  eye.  He 
obtained  in  this  .manner  a  perfectly  solid  impression, 
though  the  double  picture  occasioning  it  is  no  solid  at 
alL  Nevertheless,  some  people  are  able  to  see  stereo- 
scopic pictures  as  solids  without  using  an  instrument. 


THE   PHOTOGRAPHIC   OPTICAL    APPARATUS.  99 

But  most  persons  require  an  apparatus  which  renders  it 
possible  for  both  eyes  to  see  in  the  same  place  the  two 
pictures  that  are  separated  by  a  certain  distance.  This 
apparatus  is  the  stereo- 
scope. Its  most  essential 
features,  as  seen  in  the 
accompanying  diagram, 
are  the  double  picture  on 
the  slide,  the  partition 
on  the  interior  of  the 
box  which  prevents  the 
right  eye  seeing  the  pic- 
ture on  the  left,  and  vice  FJ°*  43' 
versa.  Further,  there  is  the  lid,  which  is  generally  pro- 
vided with  a  mirror  which  can  be  either  shut  or  opened, 
so  as  to  exclude  or  let  in  the  light  into  the  box,  and 
lastly  the  two  eye-glasses  in  the  front. 

These  eye-glasses  are  represented  in  the  diagram  as 
adjacent;  they  are  two  halves  of 
one  lens,  and  work   in  the  same 
manner.     (Fig.  44.)     We  are  in- 
debted to  Brewster  for  the  con-  Fig.  44. 
struction  of  this  instrument. 

We  have  shown  at  a  previous  page  that  a  lens  gives 
a  diminished  image  of  a  remote  object,  and  an  .enlarged 
inverted  image  of  a  near  object.  This  image  is  objective, 
that  is,  it  can  be  clearly  discerned  on  the  ground  glass 
shade  of  a  camera.  Nevertheless,  this  phenomenon  only 
takes  place  when  the  object  is  more  remote  than  the 
focus.  The  case  is  different  when  the  object  is  nearer 
to  the  lens.  Let  an  ordinary  magnifying  or  burning 
glass  be  held  near  some  writing,  and  it  will  be  seen 


100 


THE   CHEMISTRY  OF   LIGHT. 


upright,  and  not  inverted.  The  image  appears  also 
enlarged,  but  on  the  same  side  as  the  object,  and  the 
accompanying  diagram  illustrates  the  manner  in  which 

it  originates.  Thus  F 
represents  the  focus  of 
the  lens,  A  B  an  object 
within  the  focal  distance, 


-  45. 


and  a  b  its  image,  as  it 
appears  to  the  eye  upon 
the  other  side  of  the 
lens.  As  may  be  seen  in  the  diagram,  the  rays  issuing 
from  A  B  do  not  actually  unite  in  one  image,  but  their 
directions  are  thrown  back  in  the  pointed  lines  of  the 
diagram  and  lengthened  till  they  unite  in  one  point, 
and  there  we  see  the  image.  The  eye  naturally  seeks 
the  object  sought  for  always  in  the  direction  of  the 
incident  rays,  as  may  be  seen,  for  example,  in  a  mirror, 
when  we  see  the  mirrored  objects  behind  it. 


46. 


In  order  to  give  a  clear  illustration  of  the  working  of 
a  lens,  for  near  and  remote  objects,  we  introduce  the 
same  diagram  of  p.  87  again,  which  shows  the  produc- 
tion of  an  inverted  and  enlarged  image  B  A,  representing 
the  arrow  a  b  situated  within  the  focus. 

A  lens  employed  to  see  objects   enlarged  within  the 


THE    PHOTOGRAPHIC    OPTICAL   APPARATUS. 


101 


focus  is  named  a  microscope,  magnifying  glass,  or  loupe. 
These  magnifying  glasses  are  the  lenses  of  a  stereoscope. 
They  present  us  with  a  rather  enlarged  upright  image  of 
the  object  seen,  but  they  produce  likewise  the  effect  of 
a  prism.  As  may  be  seen  from  Fig.  44,  the  two  lenses 
consist  properly  of  only  two  half  lenses,  which  are  joined 
in  an  inverted  direction,  thus  making  the  impression  of 
prismatic  glasses,  and  working  similar  effects. 

We  pointed  out  at  a  previous  page  that  an  eye  o  sees 
an  object  a  through  a  prism  in 
the  direction  a  a,  that  is,  raised 
to  the  upper  side  of  the  angle 
of  refraction  of  the  prism.  The 
same  thing  happens  with  stereo- 
scopic glasses.  We  see  the  image, 
not  in  the  original  direction,  but  deflected  to  the  side 
of  the  angle  of  refraction,  that  is,  through  the  centre  of 
the  instrument. 

The  two  corresponding  points  a  a!, 
which  belong  to  the  right  and  left 
images,  appear  therefore  in  common  to 
both  eyes  at  a"  —  that  is,  at  the  same 
place  —  and  consequently  both  eyes  see 
only  one  image  instead  of  two. 

Now,  it  is  proper  to  remark  that  every 
one  who  wishes  to  see  an  object  (for 
instance,  writing)  clearly  and  distinctly 
holds  it  at  a  definite  distance  from  his 


Fig.  47. 


L 


Flg*  48' 


eye.  This  distance  is  the  distance  of  clear  vision.  In  the 
case  of  good  eyesight  it  is  eight  inches  ;  ^ith  far-sighted 
persons  it  is  more,  and  with  short-sighted  persons  it  is 
less.  In  the  case  of  stereoscopic  vision,  the  image  appears 


102  THE    CHEMISTRY   OF   LIGHT. 

remoter  or  nearer,  according  as  it  is  removed  from  or 
approached  towards  the  two  glasses.  If  the  image  is 
near  the  two  glasses,  it  appears  when  viewed  through 
them  nearer  and  smaller.  In  the  opposite  case  it 
appears  farther  and  larger.  But  every  one  wishes  to  see 
the  image  at  the  distance  of  clear  vision,  therefore 
stereoscopists  must  have  glasses  that  can  be  shifted,  in 
order  that  persons  may  adapt  the  position  of  the  image 
to  the  eye ;  that  is,  that  they  may  vary  the  distance  of 
image  and  glass  until  the  image  shows  in  the  clearest 
manner  to  them.  If  apparatus  for  this  purpose  is  want- 
ing, the  instrument  is  not  adapted  for  eyes  of  average 
power  of  vision,  and  requires  an  effort  in  eyes  of  a  diffe- 
rent calibre.  Persons  are  often  met  with  whose  eyes  are 
not  of  equal  strength,  one  being  short  and  the  other  far- 
sighted.  There  can  be  no  satisfactory  stereoscope  for 
such  cases ;  for  if  the  distance  of  the  lenses  is  adapted 
for  one  eye,  it  does  not  suit  the  other. 

Nevertheless,  such  persons  can  obtain  a  tolerable 
stereoscopic  vision  if  they  hold  a  suitable  eyeglass 
before  one  eye. 

A  great  hindrance  to  viewing  stereoscopic  pictures 
on  paper  is  the  shape  of  the  Brewster  stereoscopic  box, 
which  is  closed  all  round  and  only  jopen  at  the  top. 
This  aperture  only  admits  an  insufficient  amount  of 
light  to  the  picture,  which  is  commonly  left  in  the  shade 
on  one  side. 

This  defect  has  been  removed  in  the  American  stereo- 
scope, which  dispenses  with  any  box.  Glasses  are  fitted  in 
a  frame  g  g,  which  is  held  firm  by  means  of  a  handle  ; 
the  partition  b  serves  to  separate  the  field  of  view  of 
both  lenses.  The  image  is  placed  on  the  crossboard  with 


THE    PHOTOGRAPHIC    OPTICAL   APPARATUS. 


103 


lie  wirework  d  d,  and  this  board  can  be  easily  slid  to 
ind  fro,  so  that  the  proper  position  of  the  image  with 
reference  to  the  eye  may  be  found. 

But  the  American  stereoscope  is  only  suitable  for 
paper  pictures.  The  beautiful  transparent  stereoscopic 
pictures  on  glass  can,  however,  only  be  viewed  with 
Brewster's  stereoscope,  as  they  must  be  seen  in  light 
from  above,  and  all  light  reaching  them  from  the  front 
must  be  excluded,  or  the  effect  will  be  destroyed.  At 
Berlin,  the  firm  of  Moser  and  Company  prepares  stereo- 
scopes on  the  American  plan. 


Fig.  49. 

We  have  named  the  stereoscope  an  optic-photographic 
apparatus.  But  we  remark  that  double  pictures  drawn 
by  the  hand  can  also  be  viewed  through  it.  It  is 
evident  that  the  projection  of  such  pictures  only  succeeds 
in  very  simple  subjects.  It  would  be  very  difficult  to 
represent  stereoscopically  a  complicated  object^;  for 
example,  a  man,  a  landscape,  or  a  machine.  This  was 
only  possible  by  means  of  photography,  which  can  give 
with  the  greatest  ease  a  pictorial  reproduction  of  the 
most  complicated  objects  from  any  point  preferred.  It 
is  only  since  the  invention  of  photography  that  the 
stereoscope,  which  was  formerly  a  piece  of  furniture  in 


104  THE    CHEMISTRY   OF   LIGHT. 

collections  of  philosophical  instruments,  has  become  a 
favourite  instrument  with  the  public.  Notwithstanding 
their  small  form,  the  pictures  of  these  instruments  make 
a  clearer  and  more  intelligible  impression  than  pictures 
of  the  same  object  in  a  larger  form.  A  single  picture  of 
a  machine,  or  of  complicated  architecture  (for  example, 
the  choir  of  the  Cologne  Cathedral),  is  often  a  hopeless 
maze  of  details.  But  in  the  stereoscope  the  confused 
masses  are  directly  denned ;  they  become  distinct  in  per- 
spective, and  the  eye  perceives  with  great  clearness  the 
interior  structure.  In  this  respect  the  stereoscopic 
pictures  are  of  equal  value  to  the  magic  lantern  in 
imparting  instruction. 


CHAPTEE  XL 

THE  CHEMICAL  EFFECTS  OF  LIGHT. 

Physical  and  Chemical  Processes — Moser's  Experiments — Effects  of  Light 
npon  the  Elements — The  Manner  in  which  Phosphorus,  Oxygen,  and 
Chlorine  act  in  the  Light— Effect  of  Light  npon  Salts  of  Silver — 
Effects  of  Light  on  Chloride  of  Silver,  Bromide  of  Silver,  and  Iodide 
of  Silver — Theory  of  Development— Dry  Plates — Theory  of  the 
Positive  Process. 

IN  the  previous  chapter  we  have  become  acquainted  with 
the  part  which  light  plays  in  photographical  processes. 
We  will  now  enter  into  the  domain  of  chemistry,  in  order 
to  explain  the  phenomena  which  occur  in  consequence  of 
the  irradiation  of  light  on  substances  sensitive  to  its 
influence. 

All  bodies  in  nature  are  perpetually  subject  to 
changes.  The  sun,  moon,  and  stars  change  place ;  wood 
and  sugar  can  be  rubbed  into  dust,  and  change  form; 
lead  can  be  melted,  and  thus  altered  in  its  state  of 
aggregation.  These  modifications,  which  we  have  in- 
stanced as  illustrations,  do  not  affect  the  substance  of 
the  bodies  thus  altered.  Wood  may  be  rubbed  or  sawn 
into  the  finest  dust,  yet  it  remains  wood ;  lead  remains 
lead,  notwithstanding  the  melting  process.  *  Changes  of  • 
this  kind,  that  leave  the  substance  of  bodies  unchanged, 
are  styled  physical  changes. 


106  THE    CHEMISTRY   OF   LIGHT. 

But,  besides  these  modifications,  there  exist  others  of 
a  different  nature.  If  a  piece  of  wood  be  heated  in  flame, 
it  is  consumed ;  in  this  case  the  nature  of  wood  is  com- 
pletely destroyed.  It  becomes  changed  into  combustible 
gas,  becomes  cinder  and  ashes,  slack  and  friable,  and 
leaves  nothing  but  a  heap  of  ashes  instead  of  wood.  A 
rod  of  iron,  brought  to  a  red  heat  in  the  air,  becomes 
pliant  and  coated  with  a  black  crust,  which  falls  off  in 
powder  when  struck  by  a  hammer,  constituting  the  black 
or  magnetic  oxide.  In  this  case  the  substance  of  the 
iron  is  totally  changed.  Changes  of  this  kind  are 
styled  chemical  changes. 

Now,  light  is  able  to  produce  chemical  as  well  as 
physical  changes.  We  have  already  stated  that  the 
mineral  styled  red  sulphuret  of  arsenic  is  decomposed 
into  a  yellow  powder  when  exposed  to  the  light.  It  can 
be  melted,  in  which  case  it  forms,  on  cooling,  compact 
red  masses,  which  are  again  decomposed  on  a  further 
exposure  to  the  light.  The  number  of  physical  changes 
in  this  branch  occasioned  by  light  is  not  great,  but  the 
phenomena  are  in  themselves  remarkable. 

Moser  has  remarked  that  light  operates  on  almost  all 
surfaces.  He  covered  smoothly-polished  surfaces  of 
silver,  ivory,  and  glass  with  a  friable  coating,  and 
exposed  them  to  the  light.  After  this  he  fumigated  it 
with  vapour  of  mercury,  and  found  that  the  fumigation 
or  vapour  was  condensed  most  powerfully  where  the 
light  had  operated  on  the  surface.  Accordingly,  Moser 
established  the  proposition :  Light  works  on  all  bodies, 
-and  its  operation  can  be  made  visible  by  vapours,  which 
are  condensed  on  the  parts  exposed  to  light. 

The  chemical  changes  effected  by  light  are  far  more 


THE    CHEMICAL   EFFECTS   OF   LIGHT.  107 

numerous  than  the  physical,  and  their   study  is  the 
special  problem  of  photo-chemistry. 

Before  passing  to  complicated  phenomena  in  photo- 
graphy, we  must  make  the  reader  acquainted  with  the 
simpler  phemonena  of  the  art. 

(a)  Operation  of  Light  on  the  Elements. 

The  chemist  understands  by  the  term  elements 
simple  insoluble* bodies.  Thus  water,  which  the  ancients 
named  an  element,  is  no  element  in  the  chemical  sense 
of  the  term,  for  it  can  be  easily  decomposed  into  two 
components  of  a  gaseous  nature — oxygen  and  hydrogen. 
Air,  also  an  element  of  the  ancients,  is  no  element 
viewed  in  the  light  of  chemistry,  for  it  is  a  combination 
of  two  kinds  of  air — oxygen  and  nitrogen.  But  the  two 
latter  bodies,  oxygen  and  nitrogen,  are  undecomposable 
bodies  or  elements.  The  chemical  elements  are  the  well- 
known  metals;  also  sulphur,  phosphorus,  chlorine  (a 
greenish,  strong-smelling  gas  developed  from  chloride  of 
lime) ;  further,  the  less  known  substance  bromine  (a 
brown,  unpleasantly-smelling  substance  of  a  fluid 
nature) ;  lastly,  iodine  (a  black  substance,  also  of  a  fluid 
nature,  and  used  for  friction).  All  these  elements  unite 
together  and  produce  bodies  with  new  properties.  •  Metal 
iron  unites  with  the  light  oxygen,  and  produces  the  red 
powdery  iron  rust.  Sulphur  unites  with  oxygen,  and 
produces  the  pungent,  strong-smelling  sulphuric  acid. 
Iodine  and  chlorine  unite  closely  with  metals  forming 
the  metallic  iodides  and  chlorides,  which  have  quite 
peculiar  properties.  Amongst  these  are  ipdide  of  silver 
and  chloride  of  silver. 
It  is  remarkable  that  many  elements  present  them- 


108  THE   CHEMISTRY   OF  LIGHT. 

selves  in  quite  different  states,  so  that  it  might  "be 
supposed  they  were  quite  different  substances.  The 
yellow,  inflammable,  poisonous  phosphorus,  soluble  in 
ether,  and  formerly  used  in  the  manufacture  of  matches, 
is  changed  by  heating  in  a  closed  vessel  into  a  reddish 
substance  difficult  to  kindle,  not  poisonous,  and  insoluble. 
This  is,  however,  simply  phosphorus,  and  passes  by 
melting  into  the  state  of  common  phosphorus. 

It  is  an  interesting  fact  that  this  transformation  of 
yellow  into  red  phosphorus  is  effected  not  only  by  heat, 
but  also  by  light.  If  yellow  phosphorus  be  exposed  for 
a  long  time  to  the  light,  it  becomes  red. 

The  oxygen  of  the  air  is  also  susceptible  of  similar 
changes.  The  ordinary  oxygen  is  a  colourless  and  in- 
odorous mass.  By  the  operation  of  electricity,  however, 
it  is  easily  changed  into  another  kind  of  gas,  distinguished 
by  a  peculiar  smell — the  sulphurous  smell  attending 
lightning  when  striking  the  earth.  This  new  gas  has  a 
much  more  oxidizing  or  rusting  effect  than  common 
oxygen,  and  oxygen  thus  transformed  is  named  ozone. 

This  ozone  is  also  formed  by  the  operation  of  light : 
if  oil  of  turpentine  be  poured  into  a  large  bottle  contain- 
ing air,  and  if  it  be  agitated  violently  in  the  sunlight. 

Equally  peculiar  are  the  changes  experienced  in  the 
sunlight  by  two  other  elements  not  so  well  known, 
chlorine  and  bromine,  which  have  only  been  carefully 
observed  latterly. 

Chlorine  is  a  yellowish-green  gas,  with  a  disagreeable 
smell,  developed  by  fumigating  with  chloride  of  lime, 
and  distinguished  by  its  properties  of  destroying  or 
bleaching  coloured  stuffs,  and  annihilating  infectious 
matters.  Bromine  is  a  body  very  similar  to  it,  but  in 


THE    CHEMICAL    EFFECTS 


a  fluid  not  in  a  gaseous  state  at  an  ordinary  temperature, 
though  it  can  be  easily  vapourized,  and  then  appears 
as  a  brownish -red  gas. 

Chloride  and  bromide  gas  show  a  peculiar  relation  to 
light,  even  in  their  combinations. 

Chlorine  gas  has  a  very  peculiar  relation  to  hydro- 
gen—  a  gas  which  forms  the  chief  constituent  of 
water,  and  can  be  easily  obtained  from  it,  if  zinc  be 
thrown  in  and  diluted  sulphuric  acid  added.  When  this 
is  done,  the  zinc  attracts  the  oxygen  of  the  water,  form- 
ing, with  the  sulphuric  acid,  sulphate  of  zinc,  while  the 
hydrogen  escapes  in  the  form  of  gas. 

If  this  combustible  kind  of  gas  is  mixed  with  chlorine 
gas,  and  the  mixture  is  exposed  to  the  sunlight,  an 
explosion  takes  place.  This  accompanies  the  chemical 
combination  of  chlorine  gas  and  hydrogen  in  a  new  body 
— hydrochloric  acid — having  no  resemblance  to  chlorine 
or  to  hydrogen.  This  acid  is  of  a  sour  taste,  very  soluble 
in  water,  does  not  bleach  like  chlorine,  and  is  not  com- 
bustible. 

Another  body — iodine — is  very  closely  related  to 
chlorine  and  bromine.  Iodine  is  a  solid  body,  appearing 
in  the  form  of  shining  black  crystals,  and  emitting  when 
heated  a  wonderful  violet  vapour. 

(b)  Chemical  Effect  of  Light  on  Salts  of  Silver. 

Iodine  and  bromine  unite  with  metals  ,like  chlorine, 
forming  the  iodides,  bromides,  and  chlorides  of  metals. 
Kitchen  salt  is  one  of  the  commonest  combinations  of  this 
kind,  consisting  of  chlorine  and  sodium.  •  Sodium  is  a 
metal  not  employed  in  the  industrial  arts,  very  power- 
fully attracting  the  oxygen  of  the  air,  forming  rust,  so 


110  THE    CHEMISTRY   OP   LIGHT. 

that  it  lias  to  be  protected  by  being  kept  under  naphtha. 
The  chlorides,  bromides,  and  iodides  of  the  metals  all 
show  a  nature  analogous  to  salt.  Chloride,  bromide, 
and  iodide  of  silver  are  particularly  interesting  to  us. 
These  three  salts  are  obtained  if  we  allow  chlorine, 
bromine,  and  iodine  to  operate  directly  on  silver ;  but  a 
more  rapid  method  is  to  dissolve  in  water  chloride  of 
sodium,  bromide  of  sodium,  and  iodide  of  sodium,  and 
to  add  to  them  a-  solution  of  the  salts  of  silver. 

Silver  also  forms  salts  in  combination.  If  a  silver 
coin  is  thrown  into  nitric  acid,  it  is  dissolved,  forming 
nitrate  of  silver ;  and  this  is  obtained  after  evaporation 
as  a  white  soluble  salt. 

If  a  solution  of  this  be  mixed  with  a  solution  of  chloride 
of  sodium,  a  white  cheesy  precipitate  of  chloride  of  silver 
is  obtained  by  both  salts  interchanging  their  constituent 
parts.  Chloride  of  sodium  and  nitrate  of  silver  produce 
chloride  of  silver  and  nitrate  of  sodium. 

Bromide  of  silver  is  produced  exactly  in  the  same 
manner  if  bromide  of  sodium  be  added  to  a  solution  of 
silver,  and  iodide  of  silver  is  produced  by  adding  a  solu- 
tion of  iodide  of  sodium  also  to  a  solution  of  silver. 

Bromide  of  silver  and  iodide  of  silver  are  thus 
separated  as  cheesy  precipitates,  because  they  are  all 
three  insoluble  in  water.  If  they  are  saturated  by  being 
placed  upon  filtering  paper,  having  water  poured  upon 
them,  after  being  dried  chloride  of  silver  produces  a 
white  powder,  bromide  of  silver  a  yellowish  white,  and 
iodide  of  silver  a  yellow  powder.  All  three  are  very 
tenacious  bodies,  not  easily  decomposed  by  heat,  nor 
soluble  in  water,  alcohol,  or  ether,  but  they  can  be  dis- 
solved in  a  solution  of  hypo-sulphite  of  soda  and  cyanide 


THE   CHEMICAL   EFFECTS   OF   LIGHT.  Ill 

of  potassium,  by  combining  with  these  bodies  to  form 
new  chemical  compounds  which  are  soluble  in  water. 

These  three  combinations, — chloride,  bromide,  and 
iodide  of  silver, — which  are  peculiarly  tenacious  bodies, 
show  a  marked  sensitiveness  to  light,  and  this  sensitive- 
ness is  the  basis  of  modern  photography. 

By  the  light  of  a  gas  lamp  in  a  dark  room  the  chloride 
of  silver  appears  perfectly  white,  but  it  quickly  takes  a 
violet  tint  in  the  daylight.  It  is  often  said  that  it  becomes 
black;  this,  however,  is  not  the  case.  This  violet  colour- 
ation is  the  result  of  a  chemical  decomposition.  The 
chloride  is  liberated  and  disappears  partly  as  a  greenish 
gas,  which,  from  its  abundance  as  well  as  its  odour,  can 
be  perceived  to  be  chloride  of  silver.  The  violet  powder 
which  remains  behind  at  an  earlier  period  would  have 
bepn  taken  for  metallic  silver. 

Metallic  silver  can,  of  course,  under  certain  circum- 
stances, present  itself  in  the  form  of  a  grey  or  violet 
powder,  which  is  the  violet -coloured  body  occasioned  by 
exposing  chloride  of  silver  to  light.  But  this  is  not  metal 
silver ;  it  is  only  a  combination  of  silver  with  chlorine, 
containing  half  as  much  chlorine  as  chloride  of  silver. 
Silver  and  chlorine  form  two  combinations — one  white 
and  rich  in  chlorine;  the  other  violet,  and  with  little 
chlorine,  named  hypo-chloride  of  silver.  In  the  same 
manner,  silver  forms  two  combinations  with  bromine — one 
light  yellow,  rich  in  bromine,  named  bromide  of  silver ; 
and  a  yellowish-grey  compound,  less  rich  in  bromine, 
named  hypo-bromide  of  silver.  Further,  analogous  to 
these  there  exist  a  yellow  iodide  of  silver,  and  a  green 
hypo-iodide  of  silver,  less  rich  in  iodide.  Hypo-bromide 
and  hypo-iodide  of  silver  are  produced  exactly  in  the 


112  THE   CHEMISTRY   OF   LIGHT. 

same  manner  as  hypo-chloride  of  silver,  through  the 
operation  of  light  ;  therefore  the  chemist  says  that 
bromide,  chloride,  and  iodide  of  silver  are  reduced  to  the 
corresponding  hypo-salts. 

The  change  of  colour  by  which  this  chemical  change 
is  perceived  is  most  striking  in  chloride  of  silver,  less  in 
bromide  of  silver,  and  least  so  in  iodide  of  silver. 

It  would  appear  from  this  that  chloride  of  silver  is  the 
most  useful  to  photography.  But  the  case  is  different. 
We  have  previously  seen,  in  treating  of  the  practical 
part  of  photography,  that  plates  of  iodide  of  silver  and 
of  chloride  of  silver  are  exposed  in  the  camera.  The 
image  thus  produced  is  virtually  invisible,  but  becomes 
visible  through  a  subsequent  process,  named  the  develop- 
ing process. 

In  daguerreotypes  the  exposed  plate  of  iodide  of  silver 
was  fumigated  in  the  vapour  of  mercury.  In  this  case 
the  vapour  of  mercury  is  precipitated  in  fine  white 
globules  on  the  exposed  places,  the  amount  varying 
according  to  the  strength  of  the  light.  In  the  present 
treatment  with  collodion,  the  plate  is  washed  over  with  a 
solution  of  green  vitriol.  This  becomes  mixed  with  the 
adhering  solution  of  silver,  and  precipitates  from  it  a 
fine  black  silver  powder,  which  adheres  to  the  exposed 
places  of  the  plate. 

Therefore,  in  both  cases  we  have  a  finely  pulverized 
body,  which  is  attracted  and  retained  by  the  exposed 
places — a  mysterious  process,  as  interesting  as  it  is 
practically  important. 

From  this  it  appears  that  it  is  by  no  means  the 
colouring  of  the  salts  of  silver  which  renders  the  image 
visible,  but  the  subsequent  developing  process. 


THE  CHEMICAL  EFFECTS  OF  LIGHT.        113 

If  an  experiment  be  made  simultaneously  with 
chloride,  bromide,  and  iodide  of  silver,  by  exposing  and 
developing  them,  it  is  found  that  chloride  of  silver  gives 
the  feeblest  picture  under  the  developer,  bromide  of 
silver  a  stronger  one,  and  iodide  of  silver  the  strongest. 
Therefore,  the  very  body  which  was  most  strongly 
coloured  by  light  is  the  least  coloured  under  the 
developer,  and  the  body  which  is  the  least  coloured  by 
the  light,  viz.  iodide  of  silver,  is  the  most  coloured  under 
the  developer. 

The  developing  process  is  of  immense  importance.  If 
it  were  attempted  to  produce  a  picture  by  exposure  in  the 
camera,  without  developing,  an  exposure  of  hours  would 
be  required  before  the  impression  could  be  seen.  The 
developing  process  permits,  under  favourable  circum- 
stances, the  impression  to  become  visible  after  an 
exposure  of  only  one-hundredth  of  a  second. 

Pure  iodide  of  silver  was  formerly  used  in  photo- 
graphy, but  iodide  of  silver  is  now  used  mixed  with 
bromide  of  silver.  This  change  was  made  because  it 
was  soon  perceived  that  iodide  of  silver  is  very  sensitive 
to  strong  light,  but  by  no  means  so  to  weak  light.  For 
example,  in  taking  a  portrait,  iodide  of  silver  gives  the 
light  parts  in  a  few  seconds  with  great  clearness,  such 
as  the  shirt  and  the  face ;  whereas  the  darker  parts,  such 
as  the  shadows,  the  dark  coat,  etc.,  are  very  feebly  given. 
But,  if  some  bromide  of  silver  is  mixed  with  iodide  of 
silver,  the  coating  of  combined  iodide  and  bromide  of 
silver  gives  a  weaker  but  still  intense  picture  of  the  clear 
parts,  while  it  gives  a  much  better  impression  of  the 
dark  parts  than  iodide  of  silver  alone. 

The  mixture  of  iodide  and  bromide  of  silver  is  effected 


114  THE    CHEMISTRY   OF   LIGHT 

in  practice  by  adding  to  the  collodion  a  salt  of  iodine 
and  a  salt  of  bromine — for  example,  iodide  of  potassium 
and  bromide  of  cadmium.  Both  are  decomposed  in  the 
silver  bath.  Iodide  of  potassium  and  nitrate  of  silver 
produce  iodide  of  silver  and  nitrate  of  potash,  and  in 
the  same  way  bromide  of  cadmium  and  nitrate  of 
silver  produce  bromide  of  silver  and  nitrate  of  cadmiunic 

A  considerable  quantity  of  the  solution  of  silver 
remains  also  adhering  mechanically  to  the  collodion 
coating.  This  adhering  solution  of  silver  is  by  no 
means  a  matter  of  secondary  importance ;  on  the  con- 
trary, while  washing  in  the  developer,  it  affords  the 
material  from  which  the  fine  silver  powder  is  precipi- 
tated that  is  necessary  for  the  development. 

If  the  developer  (for  example,  a  solution  of  green 
vitriol)  is  mixed  with  a  solution  of  silver,  the  silver  is 
precipitated  in  the  form  of  a  fine  powder.  For  green 
vitriol  is  greatly  attracted  by  oxygen,  and,  taking  it  up 
readily,  passes  into  sulphate  of  iron.  Accordingly,  if  a 
body  containing  oxygen  (for  example,  nitrate  of  silver)  is 
mixed  with  green  vitriol,  the  latter  withdraws  at  once  the 
oxygen  from  the  silver  salt,  and  the  silver  becomes 
separated  by  the  process.  Other  bodies  that  readily 
combine  with  oxygen  operate  in  like  manner,  especially 
some  substances  from  the  organic  world,  such  as  pyro- 
gallic  acid,  etc.  It  was  formerly  thought  that  green 
vitriol  reduced  the  iodide  of  silver  affected  by  light,  and 
this  erroneous  opinion  is  actually  found  in  some  of 
the  most  recent  works  on  chemistry.  It  can  be  easily 
proved  that  this  view  is  false.  For,  if  a  plate  is  exposed 
and  the  nitrate  of  silver  adhering  to  it  is  washed  over, 
and  then  the  developer  poured  upon  it,  no  picture 


THE  CHEMICAL  EFFECTS  OF  LIGHT*        115 

appears,  which  proves  that  green  vitriol  alone  is  not 
able  to  operate  on  iodide  of  silver  exposed  to  light. 
But  if  a  solution  of  silver  is  added,  a  picture  appears 
immediately. 

The  solution  of  silver  adhering  to  the  plate  plays 
again  another  part.  If  a  plate  is  washed  before  it  is 
exposed — that  is,  if  all  the  nitrate  of  silver  which  adheres 
to  it  is  removed,  and  it  is  then  exposed — it  will  be 
remarked  that  it  is  far  less  sensitive  than  when  the 
nitrate  of  silver  is  present.  Whence  does  this  proceed  ? 

The  matter  is  explained  by  the  peculiar  relation  of 
many  sensitive  bodies. 

There  are  bodies  which  in  isolation  are  either  not,  or 
only  very  slightly,  sensitive  to  light,  but  which  become 
so  when  combined  in  a  compound  which  is  able  to  unite 
with  one  of  the  liberated  constituents  during  exposure  to 
light.  For  example,  chloride  of  iron  is  not  sensitive  to 
light ;  but  chloride  of  iron  dissolved  in  ether  is  sensitive 
to  light,  because  the  liberated  chlorine  unites  at  once 
chemically  with  the  ether. 

The  same  remark  applies  to  iodide  of  silver.  This  is, 
by  itself  alone,  little  sensitive  to  light ;  but  if  a  body  is 
present  which  can  combine  with  iodide,  it  is  quickly 
transformed  in  the  light.  A  body  of  this  nature  is 
nitrate  of  silver,  which  absorbs  iodine  with  the  greatest 
ease. 

This  explains  the  greater  sensitiveness  of  iodide  of 
silver  in  the  presence  of  nitrate  of  silver. 

It  follows  from  this  fact,  which  was  first  accurately 
determined  by  the  writer  of  this  book,  that  other  bodies 
which  unite  easily  with  iodine  increase  tne  sensitiveness 
of  iodide  of  silver. 


116  THE   CHEMISTRY   OF   LIGHT. 

Among  these  bodies  may  be  enumerated  extract  of 
copper,  extract  of  tea,  morphine,  tannin;  and  conse- 
quently place  in  the  hands  of  photographers  the  means 
to  prepare  what  are  called  dry  plates.  The  plates,  which 
are  prepared  in  a  silver  bath,  only  remain  moist  for  a 
short  time ;  the  adhering  solution  of  silver  dries  up,  and 
then  dissolves  the  iodide  of  silver,  so  that  the  plate  is 
actually  eaten  into.  Therefore  it  is  not  possible  to  keep 
a  supply  in  a  moist  state,  and  to  prepare  them  for  any 
length  of  time,  which  would  be  very  advantageous  in 
travelling. 

But  dry  plates  retaining  impressions  are  prepared  by 
washing  in  water,  and  removing  the  nitrate  of  silver 
adhering  to  the  moist  plate,  and  then  coating  the  plate 
with  a  solution  of  a  substance  having  relation  with 
iodine ;  for  example,  with  tannin  or  morphine.  Such 
coatings  can  dry  up  without  injury  to  the  film  of  iodide 
of  silver,  and  in  this  manner  a  dry  plate  is  obtained 
retaining  durable  impressions.  I  admit  that  the  sensi- 
tiveness of  these  plates  is  considerably  less  than  that  of 
moist  plates,  but  this  is  of  no  detriment  in  the  case  of 
objects  emitting  a  strong  light.  The  development  of  dry 
plates  of  this  kind  is  commonly  effected  with  pyrogallic 
acid.  This  is  a  substance  that  is  obtained  by  dry  distil- 
lation of  the  gall-nut.  It  operates  very  powerfully  as  a 
reducing  medium  ;  that  is,  it  precipitates  metallic  silver 
from  its  solutions,  exactly  as  green  vitriol  does. 

But  pyrogallic  acid  alone  is  not  able  to  bring  out  an 
image  on  an  exposed  dry  plate,  because  another  sub- 
stance is  required  for  this  purpose,  yielding  pulverized 
silver.  This  substance,  viz.  a  solution  of  silver,  is 
found  on  the  plates  themselves  when  they  are  moist. 


THE  CHEMICAL  EFFECTS  OF  LIGHT.        117 

But  in  the  case  of  dry  plates,  the  salt  of  silver  has  been 
washed  off;  therefore  a  mixture  of  pyrogallic  acid  and 
solution  of  silver  must  be  employed  as  developer. 
Silver  powder  is  precipitated  from  this,  and,  adhering  to 
the  exposed  places,  brings  out  the  image  into  view. 
Nevertheless  dry  plates  do  not  give  such  beautiful  and 
secure  results  as  moist  plates. 

We  have  therefore  given  an  illustration  of  the  photo- 
chemical phenomena  in  the  production  of  a  camera 
picture.  The  essential  part  of  this  process — the  negative 
process — consists  in  the  developing  of  an  invisible  light 
impression  through  a  subsequent  operation. 

But  all  pictures  are  by  no  means  prepared  in  this 
way.  We  have  already  seen,  on  the  contrary,  that  the 
pictures  on  paper  are  occasioned  by  the  production  of  a 
visible  impression  of  light,  a  piece  of  sensitized  paper 
being  exposed  until  it  is  coloured  dark.  In  this  case  no 
developing  is  required.  The  picture  is  exposed  to  the 
light  till  it  has  received  the  necessary  consistency. 

The  process  put  in  practice  in  this  case  is  quite 
simple.  The  positive  paper  contains  chloride  of  silver 
and  nitrate  of  silver.  The  former  is  quickly,  the  latter 
slowly,  reduced  by  the  light ;  that  is,  precipitated  as 
metallic  silver,  which  is  separated  as  a  brownish  powder. 
Chloride  of  silver  would  only  be  reduced  to  hypo-chloride 
of  silver.  But  by  the  presence  of  paper-fibre  the  process 
of  reduction  is  carried  further ;  it  produces  metal  silver. 
Then  the  chlorine  s'et  free  by  the  light  combines  im- 
mediately with  the  silver  of  the  nitrate  of  silver,  and 
produces  chloride  of  silver  again.  This  as  immediately 
decomposed  by  the  light.  A  fresh  quantity  of  brown 
metallic  silver  is  thus  separated,  here  again  free  from 


118 


THE    CHEMISTRY   OF   LIGHT. 


chlorine ;  and  this  process  is  repeated  as  long  as  nitrate 
of  silver  is  present,  and  as  long  as  the  light  operates. 

Pure  chloride  of  silver  alone  only  offers  a  faint  im- 
pression, but  in  contrast  with  nitrate  of  silver  it  presents 
a  very  vivid  image.  The  picture,  in  the  form  in  which 
it  is  produced  by  the  light,  is  not  durable — it  would  turn 
brown  through  the  further  operation  of  light  on  the 
white  places;  and  to  prevent  this,  the  salts  of  silver 
still  adhering  to  the  paper,  sensitive  to  light,  must  be 
removed.  The  nitrate  of  silver  is  removed  easily  by 
washing  with  water,  for  it  is  soluble  in  water ;  but  the 
chloride  of  silver  must  be  removed  by  plunging  in  a 
solution  of  hypo-sulphite  of  soda.  This  salt  becomes 
transformed  with  chloride  of  silver,  forming  chloride  of 
sodium  and  sulphate  of  silver,  and  the  latter  combines 
with  the  hypo-sulphite  of  soda  to  form  a  sulphate  of  tar- 
tar ;  and  this  sulphate,  remarkable  for  its  peculiar  sweet 
taste,  is  soluble  in  water,  and  can  be  removed  by  washing. 

If  a  fresh  impression  is  plunged  in  hypo-sulphite  of 
soda,  it  suddenly  changes  its  beautiful  violet  colour — 
it  becomes  of  a  yellowish  brown,  and  this  tint  is  not 
liked.  It  does  not  interfere  with  the  effect  in  technical 
and  scientific  pictures,  but  is  a  great  drawback  in 
portraits  and  landscapes ;  so  that  the  positive  prints 
are  subjected  to  a  further  treatment,  styled  the  colour- 
ing process.  To  this  end  it  is  plunged  in  a  very  diluted 
solution  of  gold.  This  solute  contains  chloride  of  gold. 
Metal  silver  has  more  affinity  with  chlorine  than  gold ; 
hence  it  combines  with  the  chlorine,  forming  chloride 
of  silver,  while  the  gold  is  precipitated.  It  becomes 
separated  in  the  shape  of  a  blue  colour  adhering  to  the 
outlines  of  the  picture,  and  this  blue,  mixed  with  the 
brown  of  the  picture,  gives  a  pleasant  tone,  which  does 


THE    CHEMICAL   EFFECTS   OF   LIGHT.  119 

not  change  in  the  fixing-bath, — that  is,  in  hypo-sulphite 
of  soda. 

Accordingly,  every  paper  photograph  consists  of  silver 
and  gold,  in  the  proportion  of  four  parts  of  silver  to  one 
of  gold ;  the  quantity  of  both  substances  being  very 
small.  In  a  picture  of  44  x  47  centimeters,  or  17  inches 
by  22,  only  one -thirteenth  of  a  gramme,  or  1*187  grains, 
of  metal  silver  are  contained.  Its  value  is  about  one 
German  pfennig,*  and  the  value  of  the  silver  in  a  carte 
de  visite  is  about  one-thirtieth  of  a  farthing.  The  ques- 
tion may  here  arise,  how  it  happens  that  photographers 
charge  so  high  for  their  pictures  ?  A  sufficient  reply  is 
found  in  the  fact  that  the  price  is  not  determined  by 
the  value  of  the  materials,  but  by  the  labour  which  has 
been  necessary  to  produce  the  pictures.  It  must  be  re- 
membered that  a  photographer  has  to  make  twenty- 
eight  operations  to  produce  a  negative, .  and  eight  to 
produce  a  positive ;  that  a  picture  is  often  a  failure  ; 
that  four-pennyworth  of  salts  of  silver  must  be  em- 
ployed, besides  one  farthing's -worth  of  silver  in  preparing 
a  sheet,  and  that  at  the  utmost  only  one-third  of  this 
silver  can  be  recovered  from  the  bath.  Nor  should  it  be 
forgotten  that  the  paper  itself  is  valued  at  threepence, 
that  cardboard  of  the  same  value  is  required  for  the 
mounting,  and  that  further  outlay  is  needed  for  hire 
of  premises  and  assistants — all  which  circumstances 
certainly  justify  the  price  demanded. 

If  it  is  borne  in  mind  that  thirty-three  times  as  much 
silver  must,  be  employed  as  that  which  actually  remains 
when  the  picture  is  finished,  it  will  be  ,  seen  that  the 
amount  of  silver  consumed  annually  in  photography 
must  be  enormous.  It  is  valued  at  about  £350,000. 

*  About  the  value  of  an  English  farthing. 


120  THE   CHEMISTRY   OF   LIGHT. 


CHAPTEE  XII. 

ON  THE   CORRECTNESS   OF   PHOTOGRAPHS. 

Influence  of  the  Individuality  of  the  Photographer — Different  Branches 
of  Photography — Influence  of  Lenses,  of  the  Length  of  Exposure, 
of  Colours  and  Models — The  Characteristic  Feature  in  the  Picture 
— Deviation  from  Truth  in  Photography — Difference  between  Photo- 
graphy  and  Art. 

(a)  Influence  of  the  Individuality  of  the  Photographer. 

IN  the  previous  chapters  we  have  become  acquainted 
with  the  development  and  the  theory  and  practice  of 
photography.  We  have  mentioned  cursorily  various 
practical  applications;  for  example,  the  licht-paus  pro- 
cess. It  is  our  present  purpose  to  give  our  special 
attention  to  one  point  which  is  of  great  import  in 
judging  of  the  value  of  a  photograph. 

Most  persons  have  a  fancy  that  the  application  of 
photography  is  always  uniform,  whatever  may  be  the 
object  to  be  taken,  and,  therefore,  that  a  photographer 
who  can  take  a  portrait  must  be  able  to  take  equally 
well  a  machine,  a  landscape,  or  an  oil-painting.  This 
results  from  the  erroneous  notion  that  the  picture  makes 
itself  when  the  photographer  opens  and  shuts  the  lid. 
But  our  readers  know  already  that  the  picture  does  not 
make  itself,  but  that  it  must  be  first  developed,  brought 


ON  THE  CORRECTNESS  OF  PHOTOGRAPHS.      121 

out,  fixed,  and  copied.  In  all  these  operations  there  is 
no  precise  measure  or  rule  how  long  the  photographer 
should  expose  to  the  light,  develop,  fortify,  copy,  and 
tone  the  picture.  This  depends  on  his  option  and 
judgment;  and  he  is  able  at  pleasure  to  bring  out  the 
picture  more  or  less  in  detail,  according  to  the  time  of 
exposure.  Again,  he  can  make  it  more  or  less  brilliant, 
according  to  the  degree  of  strengthening ;  he  can  make 
it  more  or  less  dark,  according  to  the  mode  of  im- 
printing; more  or  less  blue,  according  as  he  tones  it 
down.  But  what  is  it  that  directs  his  judgment  to 
determine  if  the  picture  is  correct  or  not  ?  It  is  nature, 
and  nature  alone  !  He  must  know  nature,  and  compare 
it  with  his  picture.  Nor  is  this  easy.  Nature  appears 
positive  to  him,  but  in  the  picture  she  appears  first 
negative ;  and  if  he  compares  the  two,  he  must  be  able 
in  his  mind  to  convert  the  picture,  that  is,  to  change  it 
and  represent  it  as  a  positive,  which  it  is  afterwards  to 
become.  More  comparing  and  study  are  required  to  do 
this  than  is  generally  supposed. 

If  two  printed  proofs  are  presented*  to  a  man  who  is 
ignorant  of  the  art  of  printing,  one  of  the  sheets  in 
question  being  well  and  the  other  ill  printed,  if  the 
defects  be  not  too  glaring,  this  person  will  not  be  able 
to  detect  any  difference  between  the  proofs.  Far  other- 
wise is  it  with  the  practised  eye  of  the  printer,  who 
immediately  detects  that  in  one  proof  the  type  is  too 
thick,  or  thin,  or  leaded,  or  that  the  letters  are  faint,  or 
blotched,  or  uneven.  In  like  manner,  a  practised  eye  is 
needed  to  judge  a  photograph — an  eye^not  only  able  to 
detect  the  finest  details  of  the  picture;  but  also  the 
peculiarities  of  the  original.  The  unprofessional  man 


122  THE    CHEMISTRY   OF   LIGHT. 

often  uses  the  expression,  "  I  have  no  eye  for  it," — that 
is,  "  I  am  not  accustomed  to  see  such  things," — and  it  is 
in  this  manner  that  we  first  discover  how  imperfectly  we 
use  this,  the  most  perfect  of  our  senses. 

A  man  born  blind,  and  who  recovers  his  sight  by  an 
operation,  cannot  at  first  distinguish  a  cube  from  a  ball, 
or  a  cat  from  a  dog.  He  is  not  accustomed  to  see  such 
things,  and  must  first  exercise  his  eyes  and  learn  to  see. 

We,  also,  though  in  possession  of  sound  organs,  are 
blind  to  all  things  that  we  are  not  accustomed  to  see ; 
and  this  fact  is  most  apparent  in  art,  as  also  in  photo- 
graphy, so  closely  related  to  it. 

If  photographers  principally  engaged  in  taking  por- 
traits are  not  able,  to  produce  a  good  landscape,  the 
reason  of  this  is  that  they  have  no  eye  for  landscape 
— that  they  consider  a  picture  to  be  good  after  too 
short  an  exposure,  or  when  imperfectly  developed  and 
strengthened,  or  when  inaccurately  printed.  It  proceeds 
from,  their  not  knowing  the  influence  exercised  by  the 
position  and  intensity  of  the  sun  to  the  aerial  per- 
spective produced  by  clouds,  without  speaking  of  other 
points  of  less  importance. 

Thus  every  class  of  subjects  requires  a  special  study, 
though  the  manipulation  of  photography  remains  in  all 
cases  the  same  ;  therefore,  there  are  photographers  whose 
proper  province  is  portraits,  and  others  devoted  to  land- 
scapes, to  the  reproduction  of  oil-paintings,  etc. 

(b)  Iiifluence  of  the  Object,  of  the  Apparatus,  and  of  the 

Process. 

The  remark' is  frequently  made  by  admirers  of  photo- 
graphy, that  this  newly-invented  art  gives  a  perfectly 


ON  THE  CORRECTNESS  OP  PHOTOGRAPHS.      123 

truthful  representation  of  objects,  understanding  by  the 
term  truthful  a  perfect  agreement  with  reality.  Photo- 
graphy can,  in  fact,  when  properly  applied,  produce 
truer  pictures  than  all  other  arts ;  but  it  is  not  absolutely 
true.  And,  as  it  is  not  so,  it  is  important  to  become 
acquainted  with  the  sources  of  inaccuracy  in  photo- 
graphy. Many  exist.  I  shall  treat  here  especially  of 
optical  errors. 

The  lenses  which  are  employed  in  photography  do 
not  always  give  absolutely  true  pictures.     Suppose,  for 


A 


r 


Fig.  50. 

example,  that  a  simple  lens  receives  the  impression  of  a 
square ;  it  often  represents  it  with  curvilinear  sides,  as 
in  the  accompanying  diagrams,  though  with  a  feebler 
outline.  A  picture  thrown  off  quite  out  of  drawing  by 
such  a  lens,  in  which  straight  lines  turn  out  as  curves, 
is  evidently  inaccurate.  The  inaccuracy  may  not  be 
felt  by  many,  but  it  exists.  It  may  perhaps  be  expected 
that  this  defect  disappears  in  the  case  of  what  are 
called  correct  lenses,  but  let  the  attempt  be  made  to 
obtain  a  view  with  these  correct  lenses  of  lofty  buildings 


124 


THE    CHEMISTRY   OF    LIGHT. 


taken  from  a  low  position.  The  lines  that  ought  to 
be  perpendicular  commonly  converge  upwards.  This  is 
caused  by  the  photographer  being  obliged  to  direct  his 
instrument  at  an  acute  angle  upwards,  in  order  to  be 
able  to  take  in  a  view  of  the  whole  building.  In  doing 
this,  perpendicular  lines  project  themselves,  converging 
upwards.  To  avoid  this  defect,  lenses  have  been  made 
with  a  very  large  field  of  view.  These  are  called  panto- 
scopes.  But  these  reproduce  distant  objects  apparently 


51. 


on  a  very  small  scale,  and  objects  near  at  hand  on  a 
very  large  scale, — peculiarities  unnoticed  by  unprofes- 
sional persons,  but  detected  by  close  observers  of  nature. 
A  remarkable  phenomenon,  exciting  the  wonder  of  the 
uninitiated,  is  the  distortion  of  spheres  in  photography. 
Let  the  reader  imagine  a  row  of  cannon  balls ;  these 
will  always  appear  balls  to  us,  and  the  artist  will  always 
draw  them  as  a  circle.  But  if  they  are  taken  through  a 
lens  with  a  large  field  of  view,  the  balls  situated  near 


ON  THE  CORRECTNESS  OF  PHOTOGRAPHS.      125 

the  rim  of   the   lens  no  longer   appear  circular,  but 
elliptical. 

To  explain  this  phenomenon,  we  must  attend  once 
more  to  the  mode  in  which  the  picture  is  produced.  Let 
it  be  conceived  that  there  are  three  balls  A  B  C  in  front 
of  a  camera  K,  with  the  lens  o  (Fig.  51).  Each  ball 
projects  a  cone  of  rays  on  the  optical  centre  of  the  lens. 
This  is  continued  within  the  camera,  and  cuts  the 
surface  of  the  picture,  if  its  axis  falls  obliquely  upon  it,  in 
the  form  of  an  ellipse,  such  as  A  C.  Only,  if  the  axis  of 
the  cone  of  rays  is  perpendicular  to  the  surface  of  the 
picture  S  S}  as  at  J5,  the  picture  appears  a  circle.  I  admit 
that  this  defect  only  occurs  when  the  field  of  view  of  the 
lens  is  very  large,  and  the  balls  are  situated  very  near 
its  rim. 

A  photographer  brought  to  the  author  the  picture  of 
a  castle  having  a  row  of  statues  in  front  of  it,  which  he 
had  taken  with  a  lens  having  a  large  field  of  view. 
Singularly  the  heads  of  the  statues  towards  the  margin 
became  continually  broader,  and  similarly  their  bodies ; 
and  the  slim  Apollo  of  Belvedere,  who  unfortunately 
stood  on  the  very  edge  of  the  margin,  had  such  full- 
blown cheeks  and  so  protuberant  a  paunch,  that  he 
looked  like  Dr.  Luther. 

But,  quite  independently  of  these  considerations,  there 
is  another  point  that  must  materially  affect  the  accuracy 
of  photographic  representation.  Photography  generally 
gives  the  light  parts  too  light,  and  exaggerates  the  dark 
shadows.  This  is  a  fundamental  error  which  is 
associated  with  their  very  nature,  and  which  it  is  very 
difficult  to  avoid.  It  is  seen  in  the  most  evident 
manner  in  taking  objects  lighted  by  a  brilliant  sun ;  for 


126  THE    CHEMISTRY   OF   LIGHT. 

example,  a  statue.  If  the  exposure  is  short,  a  detailed 
picture  is  obtained  of  the  light  side,  but  the  shady 
side  is  a  black  daub  or  blotch.  If  the  exposure  is  long, 
the  shady  side  is  full  of  detail,  but  the  light  side  exposed 
too  much,  and  so  thickly  covered  that  the  details  are 
wanting  in  it.  Hence  photographers  are  often  driven  to 
subterfuges  if  they  wish  to  obtain  a  correct  picture ; 
they  are  obliged  to  mitigate  the  contrasts — to  make  the 
light  more  toned  down,  and  the  shades  lighter  than 
painters  are  wont  to  make  them.  The  latter  often 
exclaim  when  they  see  the  photographic  exposure  of  a 
model,  and  wonder  if  the  picture  will  be  correct.  And 
no  doubt,  in  the  case  of  landscapes  and  architecture,  the 
results  are  not  always  satisfactory. 

The  author  once  took  a  photograph  of  the  interior  of  a 
laboratory.  It  presented  the  appearance  of  an  ordinary 
vaulted  hall.  All  was  quite  excellent.  The  tables, 
stones,  retorts,  lamps,  etc.,  were  all  seen,  only  the  vaulted 
ceiling  was  quite  dark.  New  attempts  were  made,  with 
exposures  of  20,  30,  or  40  minutes.  At  length  a  trace  of 
the  vault  appeared ;  but  now  the  objects  in  the  vicinity  of 
the  window  were  suffering  from  too  much  exposure ;  that 
is,  they  had  become  as  white  as  if  they  had  been  snowed 
over.  This  circumstance  of  photography  exaggerating 
the  dark  parts  appears  again  in  very  simple  matters, 
such  as  the  reproduction  of  copper-plates.  A  photo- 
grapher once  reproduced  a  painting  of  Kaullach's  "  Battle 
of  the  Huns."  He  produced  a  charming  photograph, 
but  the  city  in  the  background  appeared  too  thick  and 
black,  and  not  sufficiently  toned  off.  The  customer 
refused  the  photograph  and  demanded  another.  The 
photographer  made  another  attempt,  giving  a  longer 


ON  THE  CORRECTNESS  OF  PHOTOGRAPHS.      127 

exposure,  and  now  the  distance  appeared  softened  down  ; 
but,  unfortunately,  the  objects  close  at  hand,  which  had 
to  appear  black  and  clear,  turned  out  grey.  In  the  end, 
the  photographer  escaped  from  the  difficulty  by  negative 
retouche.  These  are  quite  ordinary  examples  to  show 
how  difficult  it  is  to  reproduce  an  object  correctly. 

But  we  come  now  to  the  worst  point,  that  of  colour. 
Photography  gives  the  cold  colours — blue,  violet,  and 
green — too  light,  and  the  warm  colours  too  dark.  Take 
as  an  instance  the  photographs  on  sale  of  "  Sunset  on  the 
Ganges,"  by  Hildebrandt.  It  represents  a  red  glowing 
sun  with  clouds  of  chrome  yellow  on  an  ultramarine 
sky.  But  what  becomes  of  all  this  in  the  photograph  ? 
A  black  round  disk  between  black  thunderclouds.  It 
looks  like  an  eclipse  at  Aden.  The  difficulty  of  repre- 
senting nature  is  still  more  patent  when  the  photo- 
grapher attempts  to  grapple  with  higher  artistic  ques- 
tions. Let  us  take  an  example.  There  exists  a  pretty 
genre  picture  called  "  A  Mother's  Love."  A  mother  sits 
reading  in  an  armchair ;  her  little  darling  embraces  her 
suddenly  from  behind,  and,  delightfully  surprised,  she 
drops  her  hand  with  the  book,  turns  to  look  at  her  little 
pet,  and  offers  her  cheek  to  the  little  boy  to  kiss. 

A  photographer  was  inspired  with  the  idea  of  pro- 
ducing a  similar  picture  with  the  help  of  a  living  model. 
He  found  a  comely  maiden,  who  agreed  to  personate  the 
mother,  and  a  good-looking  boy  was  also  found.  An 
armchair  for  the  mother,  a  chair,  and  other  suitable 
furniture  were  easily  procured.  The  next  point  was  the 
grouping.  The  pseudo-mother  was  very  accommo- 
dating to  the  requirements  of  the  photographer,  and 
even  assumed  a  look  which,  for  want  of  a  better,  might 


128  THE    CHEMISTRY   OF   LIGHT. 

pass  as  the  expression  of  a  mother's  love.  But  the  boy 
was  not  of  the  same  mind.  He  was  by  no  means 
attracted  by  the  pseudo-mother — he  protested  against 
coining  near  her,  and  a  good  cuff  was  needed  to  make 
him  take  up  the  requisite  position.  Time  was  thus  lost. 
The  mother  began  to  feel  uncomfortable  in  the  irksome 
position,  straining  her  neck.  The  photograph  was 
taken  at  last,  and  turned  out  sharp  and  without  spot  or 
blemish.  The  models  were  dismissed  to  their  great 
satisfaction.  What  was  the  result  ?  The  boy  was  em- 
bracing his  mother  with  a  face  bearing  evidence  of  the 
cuff  he  had  received,  and  with  a  look  as  if  he  would 
have  liked  to  murder  her ;  and  she  regards  him  with  an 
expression  that  seems  to  say,  "  Charles,  you  are  very 
unmannerly,"  and  appears  greatly  annoyed  that  her 
pleasant  reading  has  been  interrupted.  Can  it  be  said 
that  a  picture  of  this  kind  correctly  expresses  the  inten- 
tion of  the  artist?  Does  the  picture  thus  produced 
correspond  accurately  to  the  legend,  "  A  Mother's  Love"  ? 
The  untruthfulness  of  such  a  picture  will  be  evident  to 
every  one. 

Thousands  of  pictures  of  this  class  are  offered  for  sale. 
About  ten  years  a'go  errors  of  this  kind  were  committed 
by  the  thousand  in  stereoscopic  views,  and  if  they  meet 
with  approval  this  must  be  referred  exclusively  to  the 
bad  taste  of  the  public.  But  it  may  be  said  in  this  case 
it  is  not  the  photographer  who  is  guilty,  but  the  unwill- 
ing models. 

Nevertheless,  it  is  this  very  circumstance  that  throws 
such  immense  difficulties  in  the  way  of  taking  good 
photographic  portraits.  Many  persons  by  no  means 
wish  that  their  characters  should  be  correctly  given. 


ON  THE  CORRECTNESS  OF  PHOTOGRAPHS.      129 

The  rascal  wishes  to  appear  an  honourable  man  in  his 
picture ;  tottering  old  men  desire  to  appear  young, 
foppish,  and  lively  ;  the  maid-servant  plays  the  fine  lady 
in  the  atelier  ;  the  tradesman's  daughter  would  be  a 
court  lady,  the  street-sweeper  a  gentleman.  Thus  the 
picture  serves  them  only  as  a  means  of  flattering  their 
personal  vanity ;  and,  in  order  that  these  people  may 
appear  very  noble  and  distinguished,  they  put  on  a 
Sunday's  dress,  often  borrowed  and  a  very  bad  fit.  They 
practise  at  home,  moreover,  before  their  looking-glass, 
in  the  presence  of  papa,  mamma,  wife,  or  lover,  impossible 
attitudes  in  an  artistic  point  of  view.  Even  cultivated 
persons  are  not  exempt  from  these  absurdities..  Thor- 
waldsen  relates  of  Byron,  who  gave  him  a  seance,  "  He 
sat  down  opposite  to  me,  but  assumed,  immediately 
I  commenced,  a  perfectly  different  expression.  I  called 
his  attention  to  this.  '  That  is  the  true  expression  of  my 
face/  replied  Byron.  'Indeed/  I  rejoined,  and  then 
made  his  portrait  exactly  as  I  wished.  All  persons 
declared  my  bust  to  be  an  excellent  likeness.  But  Lord 
Byron  exclaimed,  '  The  bust  does  not  resemble  me ;  I 
look  much  more  unhappy/  The  fact  was  that  at  that 
time  he  wished  to  look  intensely  miserable/'  adds 
Thorwaldsen.  The  photographer  is  even  in  a  worse 
case.  If  Byron  had  come  to  a  photographer  and  had 
presented  his  face  of  misery  to  the  camera,  what  could 
the  photographer  have  done  ?  He  is  unfortunately 
dependent  on  the  model,  and  many  models  leave  him  in 
the  lurch  at  the  critical  moment,  often  not  intention- 
ally, but  from  nervousness  or  inadvertence.  Much 
depends  here  on  the  influence  of  the  photographer,  who 
must  know  how  to  control  his  sitters  with  courtesy ; 


130  THE    CHEMISTKY   OP   LIGHT. 

but  many  portraits  fail  without  any  fault  on  his  part. 
The  author  has  often  witnessed  how  persons  of  his 
acquaintance,  at  the  moment  of  being  taken,  assume 
quite  a  strange  expression  without  being  in  the  least 
aware  of  it. 

There  are  still  more  characteristic  cases  of  photo- 
graphic inaccuracy  which  cannot  be  attributed  to  the 
models.  Let  us  suppose  that  a  photographer,  stimu- 
lated by  the  beautiful  pictures  of  Claude,  Schirmer,  and 
Hildebrandt,  wished  to  photograph  a  sunset.  He 
evidently  can  only  expose  his  plate  for  a  moment  to  the 
dazzling  bright  sun.  What  sort  of  picture  is  the 
result  ?  -A  round  white  blotch  and  some  shining  clouds 
around  it.  That  is  all  that  appears  clearly.  All  objects 
in  the  landscape — trees,  houses  and  men — have  had  too 
short  an  exposure,  and  form  a  black  mass.  There, 
where  the  eye  clearly  distinguishes  road,  village,  forest, 
and  meadow,  it  sees  in  the  photograph  nothing  but  a 
dark  patch  without  any  outline.  Is  such  a  picture 
true?  Even  the  most  fanatical  enthusiast  of  photo- 
graphy will  not  dare  maintain  this. 

Such  cases,  where  violent  contrasts  of  light  and  shade 
make  the  production  of  a  correct  picture  quite  im- 
possible, are  countless  in  number.  Let  any  one 
examine  the  majority  of  the  photographs  of  the  white 
Eoyal  Monument  in  the  Thiergarten  at  Berlin.  The 
monument  is  excellently  given,  but  the  background  of 
trees  is  a  confused  black  mass,  without  details,  without 
shades  of  tone ;  the  architecture  and  other  features 
are  there,  all  except  the  splendid  foliage  that  delights 
the  eye  at  that  spot.  Still  more  numerous  are  the 
photographs  of  rooms,  in  which  the  dark  corners,  quite 


ON    THE    CORRECTNESS 


discernible  to  the  eye,  present  nothing  but  pitchy  black 
night.  There  are  other  cases  besides  these  of  photo- 
graphic incorrectness. 

Suppose  we  are  looking  at  a  mountain  landscape.  A 
small  village,  enclosed  on  both  sides  by  woody  hills, 
occupies  the  centre,  its  houses  extending  along  the 
declivities  and  scattered  picturesquely  among  the  trees. 
A  ridge  of  finely-broken  mountains  in  the  background, 
their  summits  shining  in  the  setting  sun,  frame  in  the 
wonderful  picture,  whose  effect  is  only  injured  by  one 
object — a  ruinous  pigsty  close  to  the  spectator,  with  a 
dungheap  beside  it.  A  painter,  wishing  to  paint  this 
scene,  would  certainly  have  no  scruple  about  altogether 
leaving  out  the  pigsty,  or  leaving  it  so  indistinct  and 
dark  that  it  would  not  injure  the  landscape.  But  what 
is  the  photographer  to  do?  He  cannot  pull  down  the 
offending  object.  He  seeks  another  position ;  but  there 
the  greater  part  of  the  landscape  is  concealed  by  trees. 
He  ends  by  admitting  the  pigsty,  and  what  kind  of 
picture  is  the  result  ?  On  account  of  its  vicinity,  the 
pigsty  appears  of  colossal  size  in  the  picture.  On  the 
other  hand,  the  landscape,  which  is  the  principal  thing, 
appears  small  and  inconsiderable.  A  still  more  fatal 
adjunct  is  found  in  the  dung-heap  occupying  almost 
one-fourth  of  the  picture.  As  the  most  brightly  lighted 
part  of  the  photograph,  it  immediately  attracts  the  eye  of 
the  beholder ;  it  diverts  his  glance  from  other  important 
points ;  it  acts  as  a  disturbing  influence.  The  photo- 
graph obtained  does  not  appear  as  a  picture  of  the 
landscape,  as  it  ought  to  be,  but  as  a  view  of  the  pig- 
sty. The  accessory  has  become  the  principal  point. 
The  picture  is  untrue.  It  is  untrue,  not  because  the 


132  THE    CHEMISTRY   OF   LIGHT. 

objects  it  represents  were  not  present  in  nature,  but 
because  the  accessories  are.  presented  too  glaringly  and 
too  large,  while  the  principal  parts  appear  too  small, 
indistinct,  and  inconsiderable. 

This  brings  us  to  a  weak  point  in  photography,  which 
represents  accessories  and  principal  features  as  equally 
defined.  The  plate  is  indifferent  to  everything,  while 
the  genuine  artist,  in  reproducing  a  view  of  nature,  gives 
prominence  to  what  is  characteristic,  and  entirely  keeps 
under  or  softens  off  accessories.  He  can  dispose  and 
manage  it  with  artistic  freedom,  and  he  has  a  perfect 
right  to  do  so,  because,  by  his  giving  prominence  to 
what  is  characteristic,  and  dropping  what  is  accessory, 
he  is  truer  than  photography,  which  gives  equal 
prominence  to  both,  and  often  more  to  what  is  accessory. 
Eeynolds  says  of  the  portrait  of  a  lady  in  which  an  apple- 
tree  was  most  carefully  painted  on  the  background : 
"  That  is  the  picture  of  an  apple-tree  and  not  of  a  lady." 
Similar  remarks  might  be  made  on  seeing  many  photo- 
graphs. It  is  a  cardinal  error  in  their  case,  that  they 
give  a  stronger  tone  to  accessories  than  to  essentials. 
They  present  a  conglomerate  of  furniture,  and  it  is  only 
after  careful  inspection  that  a  man  is  detected  sticking 
among  it,  whose  portrait  is  to  form  the  picture.  In 
another  case  a  quilted  white  blouse  is  seen,  and  it  is  only 
after  some  time  that  a  girl's  head  is  perceived  rising- 
above  it.  A  park  is  seen  in  a  landscape,  with  fountains 
and  other  adornments,  and  it  is  only  after  some  time 
that  a  black  coat  is  seen  confounded  with  an  equally 
dark  bush. 

It  may  perhaps  excite  surprise  that  the  writer 
ascribes  greater  truth  to  painting  than  to  photography, 


ON  THE  CORRECTNESS  OF  PHOTOGRAPHS.      133 

which  is  generally  regarded  as  the  truest  of  all  methods 
of  producing  pictures.'  It  must  be  self-evident  that  the 
remark  has  only  been  made  in  connection  with  works  of 
the  first  masters.  One  of  the  great  services  of  photo- 
graphy is  that  it  has  rendered  impossible  those  daubs  of 
inferior  artists  formerly  offered  for  sale  in  every  street. 
But  the  perfect  picture  of  the  photographer  is  not  self- 
created.  He  must  test,  weigh,  consider,  and  remove  the 
difficulties  which  oppose  the  production  of  a  true 
picture.  If  his  picture  is  to  be  true,  he  must  take  care 
that  the  characteristic  is  made  prominent  and  the 
accessories  subordinate.  The  non-sensitive  plate  of  iodide 
of  silver  cannot  do  this.  It  receives  the  impression  of 
all  that  it  has  before  it,  according  to  unchangeable  laws. 
The  photographer  attains  this  end,  on  the  one  hand,  by 
appropriate  grouping  of  the  original ;  on  the  other,  by  a 
proper  treatment  of  the  negative.  I  admit  that  to  do 
this,  he  must  also  be  able  to  detect  what  is  characteristic 
and  what  accessory  in  his  original. 

Therefore,  whoever  wishes  to  undertake  any  photo- 
graphic production  must  first  become  familiar  with  the 
object  that  he  wishes  to  take,  that  he  may  know  what  he 
has  to  do.  The  photographer  will  not,  indeed,  be  able  to 
control  his  matter,  like  the  painter,  for  the  disinclina- 
tion of  models  and  the  optico-chemical  difficulties  often 
frustrate  his  best  endeavours,  and  hence  there  must 
always  be  a  difference  between  photography  and  a  work 
of  art.  This  difference  may  be  briefly  summed  up  by 
saying  that  photography  gives  a  more  faithful  picture 
of  the  form,  and  art  a  more  faithful  picture  of  the 
character. 


• 


134 


THE   CHEMISTRY   OF   LIGHT. 


CHAPTEE  XIII. 

LIGHT/SHADE,  AND  PERSPECTIVE. 

The  Difference  between  the  Picture  and  Reality— Effect  of  Shade — 
Perspective  Foreshortenings — Effect  of  the  Position  of  the  Spec- 
tator— Influence  of  Distance — Influence  of  the  Eye-line. 

IN  the  previous  chapter,  while  treating  of  the  incorrect- 
ness of  photographs,  we  tacitly  took  for  granted  that  it 

o  is  possible  to  give  a  true 
picture  of  an  object,  if  not 
by  photography,  by  the  hand 
of  a  skilful  artist. 

We  will  now  see  how  far 
this  assumption  is  admis- 
sible. 

Let  the  simplest  case  be 
taken ;  for  example,  a  cube 
or  a  cylinder.  Let  these  be  drawn,  and  figures  will  be 
obtained  nearly  identical  with  those  marked  X  and  S  in 
the  diagram.  Now,  these  figures  are  flat  like  the  paper, 
while  the  originals  are  solids.  It  may  be  said  that 
picture  and  solid  agree ;  but  it  is  not  so.  Let  a  blind 
man  be  questioned,  who  knows  the  bodies  by  touch. 
Now,  the  cube  can  be  moulded  in  marble  or  gypsum. 


Fig.  52. 


LIGHT,    SHADE,    AND   PERSPECTIVE. 


135 


A 


In  this  case,  the  deception — for  such  it  is — can  be 
carried  to  great  lengths.  The  wood  of  the  cube  or  of 
the  cylinder  can  be  imitated  by  painting.  The  "eye  will 
readily  pronounce  such  imitation  to  be  wood.  The 
blind  man,  who  feels  both,  will  say :  The  form  agrees, 
but  not  the  mass — one  cube,  that  of  wood,  feels  warm  ; 
the  other,  that  of  stone,  cold. 

The  principles  that  apply  to  these  two  objects  apply 
to  all  objects  and  their  representations.  None  of  them 
is  a  perfectly  true  copy  of  the  object.  When  the  sur- 
face representation  makes  on  our  eye  the  impression  of 
a  solid  object,  this  is  a  deception,  by  which  our  eye  suffers 
itself  to  be  deceived. 

If  two  rectangles  are  drawn, 
A  and  B,  on  paper,  both  appear 
as  plane  figures.  But  directly 
the  rectangle  B  is  shaded  with 
thinner  or  thicker  lines,  the  rect- 
angle no  longer  appears  such, 
but  a  spherical  body.  Thus,  by  Fig.  53. 

imitating  the  gradations  of  light  and  shade,  we  have 
produced  a  deception  for  our  eye,  and  this  aid — the 
division  of  light  and  shade — is  one  of  the  most  important 
distinctions  in  art,  to  give  a  spherical  appearance  to 
flat  surfaces. 

But  there  is  another  and  a  more  important  means  of 
deception — perspective. 

If  we  consider  a  cube  (Fig.  52)  whose  faces  are  of 
equal  length,  we  perceive  that  these  faces  appear  of  very 
different  length.  The  surface  turned  towards  our  eye 
appears  a  square,  while  the  others  are  shortened  in  a 
marked  degree,  the  surface  appearing  quite  irregular, 


136  THE    CHEMISTRY   OF   LIGHT. 

the  parallel  lines  running  into  one,  and  converging  to 
one  point  o,  called  the  vanishing  point  (Fig.  52). 
The  same  thing  happens  with  all  other  bodies :  a 
pendant  human  arm  or  a  standing  column  S  (Fig.  52) 
appear  at  their  full  length,  but  the  lying  column  L  and 
the  arm  extended  towards  us  appear  foreshortened. 
Their  dimensions  are  contracted ;  in  short,  we  see,  instead 
of  the  shaft  of  a  column,  only  its  circular  base  6,  and 
this  again  appears  sometimes  round,  when  its  full  surface 
is  turned  towards  us,  at  others  an  ellipse,  which  it  is  not 
in  fact,  and  in  this  case  the  parallel  sides  of  the  column 
run  into  one.  The  track  of  a  railroad  viewed  in  per- 
spective presents  the  'same  features.  The  fact  that  we 
do  not  feel  this  deception — for  such  it  is — to  be  one, 
results  from  habit. 

We  know  from  experience  that  the  arm  extended 
towards  us  is  longer  than  it  appears  in  perspective,  to 
our  eye,  and  also  that  the  rails  which  appear  to  run 
together  are  parallel.  We  are  continually  correcting  the 
errors  of  our  visual  organ.  Accordingly,  the  eye  gives 
us  a  false  representation  of  objects,  and  the  painter 
takes  advantage  of  this  circumstance.  He  represents 
the  lying  column  L  b,  and  the  retiring  sides  of  the  cube, 
as  falsely  as  we  see  them — that  is,  "  foreshortened  "  in 
their  dimensions,  running  together  in  their  parallel  lines 
— and  every  one  is  deceived  by  this. 

It  is  the  province  of  the  artist  as  of  the  photographer 
to  represent  perspective  correctly,  that  is,  as  it  appears 
to  our  eye.  If  this  is  not  the  case,  the  picture  appears 
incorrect. 

Perspective  teaches  us  the  law  of  foreshortening. 

Our  eye  is  a  camera  obscura  with  a  simple  landscape 


LIGHT,    SHADE,    AND   PERSPECTIVE.  137 

lens.  It  is  known,  from  optics,  that  the  representa- 
tion of  a  point  lies  on  the  straight  line  drawn  from 
the  point  to  the  optical  centre  of  the  objective.  The 
representation  of  the  point  is  at  the  place  where  this 
line,  named  the  principal  radius,  cuts  the  plane  of  the 
image — the  ground  glass  shade  of  the  camera,  or  the 
retina  of  our  eye.  Accordingly,  the  representation  of  a 
straight  line  is  the  place  where  the  radii  from  the 
separate  points  of  the  line,  passing  through  the  optical 
centre,  cut  the  ground  glass  table  or  shade.  Now,  these 
radii  form  a  plane,  and  this  plane  cuts  the  flat  table  of  the 
picture  in  a  straight  line.  Therefore  the  picture  of  a 


Fig.  54 

straight  line  is  to  our  eyes  another  straight  line,  and  the 
image  of  a  plane  triangle  another  plane  triangle.  If 
the  flat  figure  is  parallel  to  the  retina,  that  is,  to  the 
table  of  the  picture,  by  well-known  stereometric  laws  the 
representation  is  like  the  original.  Let  the  reader 
imagine  a  glass  slab  placed  perpendicular  to  the  axis  of 
his  eye ;  then  the  rays  or  pencils  of  light  abed 
issuing  from  this  object  will  cut  it  so  as  to  form  a 
figure  a'  V  c'  d!  (Fig.  54).  If  such  a  figure  is  constructed 
for  a  given  point  of  intersection  and  a  given  canvas, 
this  drawing,  if  brought  to  a  proper  position  and  dis- 
tance from  the  eye,  will  produce  on  it  exactly  the  same 
impression  as  the  object  itself.  This  is  the  secret  of  the 


138 


THE    CHEMISTRY   OF   LIGHT. 


deception  that  a  plane  picture,  properly  constructed,  can 
appear  spherical.  A  picture  designed  in  the  manner 
just  described  is  named  a  drawing  in  perspective.  It  is 
evident  that  such  a  drawing  must  be  viewed  under  the 
same  conditions  as  those  in  which  it  was  designed. 

If  A  B  C  D  (Fig.  55)  is  the  outline  of  a  house,  B  the 
canvas,  0  the  point  of  intersection  of  the  rays,  abed 
the  representation  of  the  points  A  B  C  D,  the  eye 
must  be  brought  exactly  to  the  point  of  intersection  0  if 
the  representation  in  perspective  a  b  c  d  is  to  produce 
the  same  impression  as  the  object. 

If  the  canvas  is  brought 
nearer  to  the  eye  (for  example, 
to  B),  it  is  evident  that  the  rays 
will  intersect  at  a  very  different 
angle  from  those  issuing  from 
the  object  A  B  C  D;  accord- 
ingly, they  cannot  produce  a 
correct  impression.  The  same 
thing  would  be  the  case  if  the 
Flg>  55*  canvas  wrere  removed  farther 

from  the  eye  (e.g.  to  B").  Therefore  every  drawing  in 
perspective  must  be  viewed  from  the  point  of  intersection 
of  the  rays  adopted  as  the  basis  of  its  construction,  if 
it  is  to  produce  a  correct  impression. 

Now,  photography  is  a  drawing  in  perspective  whose 
point  of  sight  is  in  the  objective.  Accordingly  the  inspect- 
ing eye  must  be  brought  to  the  same  distance  as  the 
objective,  that  is,  to  the  focal  distance.  If  this  is  not 
done,  the  impression  is  untrue. 

We  have  lenses  with  a  focal  distance  of  only  four 
inches,  and  even  less  ;  and  at  such  a  distance  it  is 


LIGHT,    SHADE,    AND   PERSPECTIVE.  139 

impossible  to  see  a  drawing  with  the  unaided  eye.  To  do 
this,  it  must  be  held  at  the  distance  of  at  least  eight 
inches,  and  that  is  the  reason  why  photography  in  such 
cases  produces  an  untrue  impression.  Such  cases  fre- 
quently occur  when  views  are  taken  with  divergent  lenses. 
There  are  other  abnormal  appearances  which  accom- 
'  pany  portrait  taking.  Thus,  the  same  object  presents 
an  entirely  different  picture  according  as  it  is  viewed 
far  or  near.  Let  the  reader  conceive  a  pillar  with  the 
outline  A  B  C  D,  let  it  be  viewed  from  P;  in  this  case 
the  faces  A  B  and  C  D  will  be  perfectly  seen.  Now  let 
the  spectator  approach  nearer  to  the  object,  for  example, 
to  O.  From  this  position  nothing  is  any  longer  seen 
of  the  faces  ;  the  entire  character  of  the  picture 
becomes  changed.  If  instead  of  a  pillar  a  human  face 


:— — 

lH  ~~^~  -l~~~ , 


Fig.  56. 

be  thought  of,  it  is  evident  that  the  cheeks  will  contract 
if  we  approach  the  object,  and  the  face  appear  too 
narrow  in  proportion  to  its  height. 

The 'accuracy  of  this  conclusion  is  proved  by  the 
following  illustration.  These  are  two  representations 
of  the  head  of  Apollo  at  the  distance  of  47  and  112 
inches.*  The  bust  was  placed  perfectly  upright,  also 

*  In  order  to  secure  a  correct  reproduction  *of  both,  they  were 
'reproduced  upon  wood  by  a  process  of  photographic  wood-cutting.  I 
admit  that  the  reproduction  does  not  give  the  powerful  impression  o±  tne 
original,  but  it  is  sufficiently  intelligible  to  the  careful  observer. 


140 


THE    CHEMISTRY    OF    LIGHT. 


the  photographic  apparatus,  and  the  directing  line  was 
most  carefully  measured. 


The  contrast  is  obvious.     The  whole  figure  appears  in 
I.  slimmer,  the  chest  almost  contracted;   on  the  other 


LIGHT,    SHADE,    AND    PERSPECTIVE.  141 

hand,  the  same  model  II.  appears  with  full  cheeks  and 
square  shoulders.  That  this  slimness  is  by  no  means  a 
mere  deception  of  the  eye  is  certain  from  the  best 
measurement.* 

The  distances  between  the  eye  and  the  point  on  the 
chest  marked  by  a  cross  are  exactly  equal  in  both  busts 
— the  greatest  breadth  of  chest,  including  the  ends  of 
both  arms,  amounts  in  I.  to  2*29472  inches,  and  in  II. 
to  2*32283  inches.  Quite  independently  of  this  glaring 
difference,  there  are  other  marked  distinctions  between 
the  two  heads  which  strike  the  careful  observer.  Let  a 
line  a  a  be  drawn  across  the  top  of  the  hair — in  II. 
it  is  horizontal,  in  I.  it  inclines  to  the  right. 

Next  let  attention  be  directed  to  the  pedestal.  The 
curves  in  I.  are  strongly  inclined,  and  in  II.  are  quite 
horizontal. 

Let  the  ends  of  the  arms  A  A  be  next  considered. 
In  I.  the  side  surface  is  scarcely  seen,  and  in  II.  it  is  very 
apparent.  In  like  manner,  it  is  clearly  seen  that  the 
back  pediment  at  u  in  II.  stands  out  more  than  in  I.  In 
II.  the  head  stands  more  between  the  shoulders  (let  the 
angle  of  the  neck  be  observed  at  W),  in  I.  it  rises  up 
more ;  therefore  the  whole  form  appears  in  I.  to  raise  up 
the  head  more  on  high.  In  II.  the  head  appears  some- 
what bent  forward  ;  and  yet  the  figure  was  immovable, 
the  lenses  employed  free  from  flaw,  the  eye-line  and 
height  were  the  same  in  both.  Nothing  was  different  but 
the  distance. 

The  author,  besides  taking  these  two  heads,  has- taken 
two  others  at  the  distance  of  60  and  80  inches ;  and  if 

*  In  the  original  photograph,  where  the  two  busts  stand  out  from  a 
black  background,  this  difference  is  still  more  marked. 


142  THE    CHEMISTRY   OF   LIGHT. 

the  four  heads  thus  taken  are  placed  beside  each  other, 
it  is  seen  how  with  the  increase  of  distance  the  form 
becomes  thickset,  fuller,  and  dumpy;  how  the  hair 
sinks  more  and  more ;  how  the  ellipses  become  closer  ; 
and  how  the  chest  increases  in  width,  and  the  stumps  of 
the  arms  widen  out. 

Thus,  therefore,  we  see  very  different  views  of  the 
same  object  at  different  distances;  just  as  the  same 
portrait  placed  in  different  lights  expresses  an  entirely 
different  character. 

It  may  be  objected  that  these  are  small  matters,  and 
that  it  is  indifferent  whether  it  looks  a  little  too  thin  or 
too  fat.  To  many  this  may  appear  unimportant  in  the 
case  of  Apollo — most  persons  do  not  know  in  the  least 
how  he  looks.  But  it  is  a  different  matter  in  the 
photography  of  portraits,  where  the  highly  distinguished 
personality  of  the  customer  is  in  question.  Persons 
quite  untutored  in  art  have  a  very  quick  eye  where  their 
own  physiognomy  is  in  question — a  line,  a  wrinkle,  an 
outline,  a  curl,  are  in  this  case  criticised,  and  differences 
that  would  not  be  at  all  remarked  in  the  images  of  Apollo 
become  very  striking.  It  is  therefore  the  affair  of  the 
photographer  to  attend  to  the  effects  of  distance. 

Now,  many  persons  would  perhaps  wish  to  know, 
which  distance  is  the  best  ?  which  gives  the  most  correct 
picture  ? 

We  might  reply  that  this  depends  on  the  individuality 
of  the  person.  Painters  in  general  recommend  for  the 
drawing  of  an  object  a  distance  that  is  twice  its  own 
length  ;  accordingly,  for  a  man  five  German  feet  in 
height,  a  distance  of  about  ten  feet ;  for  his  bust,  about 
five  feet.  The  painter,  however,  has  here  greater  free- 


LIGHT,    SHADE,    AND    PERSPECTIVE. 


143 


dom     he  can  add,  omit,  and  change  at  his  pleasure.     In 
photography  this  is  only  partially  possible. 

Just  as  elevated  solid  bodies  appear  different  at 
different  distances,  hollow  shapes  also  appear  different 
at  different  distances. 


xs 


7) 


Fig.  5S. 


If  A  B  C  D  (Fig.  58)  is  the  inside  of  a  box,  we  see 
the  side  A  B  from  P  much  more  foreshortened  than 


Fig.  60. 

from  0'  to  N ;  therefore,  if  taken  under  similar 
relations,  from  near  and  at  a  distance,  it  will  appear 
wider  in  proportion  to  its  height  in  the  former  case. 


144  THE    CHEMISTEY    OF    LIGHT. 

This  state  of  things  occurs  if  we  imagine  A  C  to  be  the 
trunk,  and  C  D  the  lap  or  the  feet,  of  a  seated  person. 
In  that  case  the  lap  appears  much  larger  in  relation  to 
the  trunk,  and  the  feet  of  a  standing  person  facing  you 


Fig.  61. 

appear  longer  from  N.  Let  the  reader  observe,  for 
instance,  the  foot  of  the  Apollo  in  I.  (Fig.  57),  which  is 
much  more  prominent  than  in  II.  Lastly,  let  C  D 
(Fig.  58),  be  supposed  to  be  the  carpet  or  ground;  this 


LIGHT,    SHADE,    AND   PERSPECTIVE.  145 

will  appear  wider — that  is,  rising  higher — seen  from  N. 
Therefore,  if  the  same  person  is  taken  from  different 
positions,  P  and  0',  so  that  the  height  of  the  body 
remains  the  same  in  both  pictures,  in  that  one  taken  at 
a  shorter  distance  the  prominent  parts — lap,  hands,  and 


Fig.  62. 


feet — appear  wider,  and  the  ground  or  chair  more 
inclined  (Fig.  59)  than  in  the  picture  taken  from  P. 

Very  essential  changes  result  from  an  alteration  in 
the  height  of  the  spectator's  eye. 

If  a  standing  person  is  looked  up  to,  so  that  the  head 


146  THE    CHEMISTRY    OF    LIGHT. 

of  the  spectator  is  lower  than  the  head  of  the  object,  the 
latter  appears  thrown  back.  If  the  head  of  the  spectator 
is  on  a  level  with  the  head  of  the  object,  the  latter 
appears  perpendicular ;  if  the  spectator  is  higher,  the 
head  of  the  object  appears  inclined  forward. 


Fig.  63. 

The  three  accompanying  diagrams,  taken  from  photo- 
graphs, will  make  this  evident.  The  first  shows  the  view 
taken  from  a  level,  the  second  the  view  taken  from 
above,  the  third  the  view  from  below. 

Similar  differences  occur  in  viewing  a  landscape  from 


Fig.  66. 


148  THE    CHEMISTBY   OF   LIGHT. 

a  high  and  from  a  low  position,  as  may  be  seen  from  the 
three  accompanying  woodcuts.  The  dotted  horizontal 
line  shows  the  height  of  the  eyes  of  the  looker-on  (his 
horizon).  The  first  picture  gives  a  view  as  a  person 
sitting  on  the  ground  would  see  it :  the  milestone  on  the 
left  appears  unusually  high,  towering  to  the  sky,  and 
the  men  appear  taller,  but  the  ground  looks  contracted 
(foreshortened).  The  second  picture  gives  the  view  as 
seen  by  a  man  standing  erect :  in  this  case  the  ground 
widens  out,  rising  higher,  and  the  milestone  appears 
lower.  In  the  third  picture,  which  gives  the  view  from 
twice  the  height  of  the  man,  the  figures  and  the  mile- 
stone appear  small  and  contracted.  You  look  down  upon 
them  as  on  persons  who  are  smaller  than  the  spectator, 
while  the  ground  widens  out  and  rises  considerably  in  the 
picture.  These  examples  show  how  important  is  the 
choice  of  the  stand-point  both  in  photography  and 
painting,  and  how  an  incorrect  choice  of  it  produces 
quite  an  abnormal  view  of  the  objects.  The  photographer 
is  unfortunately  often  obliged  to  take  a  position  that  does 
not  give  a  favourable  view,  for  example,  of  lofty  build- 
ings in  narrow  streets  (the  Eathhaus  in  Berlin,  St. 
Stephen's  at  Vienna),  or  among  mountains,  where  often 
the  trunk  of  a  tree,  which  did  not  offend  the  eye  of  the 
spectator,  destroys  the  picture  of  the  photographer,  and 
forces  upon  him  the  choice  of  a  less  picturesque  but  un- 
incumbered  locality. 


CHAPTEE  XIV. 

THE  APPLICATIONS  OF  PHOTOGRAPHY. 

WE  have  by  this  time  learnt  to  know  the  difficulties 
which  oppose  the  obtaining  a  correct  photographic 
picture;  and  now  the  reader  will  more  easily  understand, 
if  we  examine  with  greater  minuteness  the  different 
problems  the  solution  of  which  photography  has  up  to 
the  present  time  attempted. 

We  shall  prolong  this  examination  only  so  far  as  it  is 
useful  for  the  comprehension  of  the  subject,  and  as  it  is 
of  general  interest  to  every  one. 

SECTION  I. — PORTRAIT  PHOTOGRAPHY. 

Popularity  of  the  Portrait  Branch — JEsthetical  Defects — Dependence  of 
Success  on  the  Person  to  be  taken — Effect  of  Dress — Effect  of 
Colours — Pictures  of  Children  and  in  Groups — Effect  of  the  Size 
of  the  Picture — Life-size  Pictures — Momentary  Pictures — Photo- 
graphic Copies  of  Photographs. 

Scarcely  any  other  branch  of  photography  has  enjoyed 
so  much  popularity  as  that  of  portrait  taking.  Most 
persons  conceive  under  the  term  photographer  only  a 
portraitist,  and  only  few  are  aware  that  .photography  is 
good  for  something  else.  The  photographic  portrait 
owes  its  popularity  to  its  extraordinary  cheapness,  to 


150  THE    CHEMISTRY   OF   LIGHT. 

its  rapid  mode  of  execution,  and  to  its  relatively 
greater  resemblance  when  compared  with  drawings 
from  nature.  Imperfect  photography  can  reckon,  on 
account  of  these  circumstances,  on  greater  support  than 
the  drawing  of  a  clumsy  artist ;  and  the  more  so  as  a 
false  conceit  exists  that  photography  must  have  un- 
deniably more  likeness  to  the  original— which  is  by  no 
means  the  case,  as  we  have  seen. 

Photography  has  driven  mechanical  portrait-painting 
out  of  the  field — only  the  genuine  artist  is  able  to  hold 
his  ground  against  it.  Portrait -photography  makes 
greater  demands  than  any  other  branch  on  the  good 
taste  of  the  photographer,  his  capacity  to  give  a 
natural,  or  at  least  apparently  natural,  picturesque,  un- 
studied pose  to  a  person.  It  requires  him  to  show  the 
best  side  of  the  sitter,  to  conceal  as  far  as  possible  any 
defect  that  may  exist,  to  bring  out  all  that  is  advan- 
tageous, and  by  a  clever  adaptation  of  light  to  give 
prominence  to  the  essential  points  and  leave  indistinct 
those  parts  which  would  injure  the  effect  of  the  picture. 
To  this  end  the  photographer  is  at  liberty  to  take  in  the 
surroundings  of  the  person,  be  they  a  chamber  or  a 
landscape,  or  to  exclude  them  by  screens. 

In  the  first  period  of  photography  the  pictures  were 
commonly  crammed  with  accessories,  and  incredible 
errors  were  committed  with  relation  to  position  and 
light ;  but  now  the  more  advanced  photographers  have 
taken  hints  from  artists,  and.  latterly  pictures  are  seen 
which,  notwithstanding  the  mechanical  character  in- 
herent in  their  production,  create  quite  an  artistic  effect. 

The  model — that  is,  the  person  to  be  taken — has  a  very 
essential  share  in  the  work  of  photography.  Not  un- 


THE   APPLICATIONS   OF   PHOTOGRAPHY.  151 

frequently  persons  go  to  a  photographer  in  a  very 
morose  frame  of  mind,  or  they  lose  their  patience 
through  some  cause  or*  other  of  delay;  it  also  often 
happens  that  people  go  to  him  with  some  slight  malaise, 
with  headache,  or  a  restless  night.  This  is  a  great 
mistake;  the  bodily  or  mental  condition  stamps  itself 
infallibly  on  the  picture,  and  often  gives  it  a  very 
dissimilar  expression  to  the  original,  even  after  the 
photographer  has  used  all  his  art  upon  it.  In  like 
manner,  it  very  frequently  happens  that  persons  at  the 
moment  of  being  photographed  put  on  a  perfectly 
strange  expression,  force  a  smile,  stare,  let  the  mouth  fall 
open,  or  are  disturbed  by  the  iron  props  which  keep  the 
head  in  place  and  are  quite  essential  if  a  well-defined 
picture  is  proposed,  but  which  are  only  submitted  to 
under  protest  by  the  model,  who  fancies  he  can  sit 
motionless  without  such  aid. 

None  of  these  influences  can  be  set  aside  by  the  photo- 
grapher. The  persons  who  present  themselves  to  have 
their  portraits  taken  are  for  the  most  part  unknown  to 
him.  He  has  often  only  five  minutes  to  study  the 
faces  of  the  persons,  to  find  their  best  side,  to  make 
them  pose,  and  to  place  them  in  accord  with  the  sur- 
roundings. He  probably  attends  to  these  matters  as 
skilfully  as  any  one,  yet  he  has  no  power  over  the 
features  of  the  original.  Nor  has  he  any  conception  if 
the  expression  of  a  person  is  his  usual  one,  or  if  it  be 
modified  by  ill  temper  or  ill  health.  In  the  latter  case 
it  is  impossible  for  the  picture  to  please,  however 
masterly  may  be  the  execution;  yet  in  ftiis  case  the 
fault  lies  not  with  the  photographer,  but  with  the 
original. 


152  THE    CHEMISTRY   OF   LIGHT. 

Another  cause  of  failure  is  the  inclination  of  many 
persons  to  choose  their  own  position,  whether  of  their 
own  accord  or  prompted  by  friends.  These  attempts 
generally  fail,  because  the  errors  in  perspective  pre- 
viously enumerated  are  overlooked.  People  in  general 
do  not  know  this,  but  the  photographer  does.  Other 
inconveniences  result  from  the  very  nature  of  photo- 
graphy. Blue  eyes  are  generally  too  light  and  dim, 
blonde  hair  too  dark,  and  auburn  hair  reddish.  Many 
of  these  difficulties  are  removed  by  clever  negative 
retouches,  but  by  no  means  all. 

Still  greater  are  the  hindrances  which  the  toilette  and 
the  changefulness  of  fashion  furnish.  Bad  taste  of  the 
person  is  much  worse  and  more  visible  in  photography 
than  in  nature.  Ladies  deficient  in  taste  are  in  the 
habit  frequently  of  making  their  necks,  which  are 
naturally  short,  still  shorter  and  thicker  by  necklaces. 
They  disfigure  a  form,  perhaps  naturally  excellent,  by 
long  trains ;  the  back  of  their  head  by  an  ill-assorted 
chignon ;  and  their  hair  by  ribbons  of  the  brightest  and 
most  glaring  colours.  In  these  cases  the  photographer 
can  do  much  service  by  his  good  advice.  The  difficulties 
are  even  increased  if  groups  of  persons  or  children,  and 
not  individuals,  are  in  question. 

The  children  must  be  amused  by  deceptions.  If  the 
photographer  wishes  to  succeed  in  taking  children's 
portraits,  he  must  know  how  to  win  the  confidence  of  the 
little  people ;  this  is  the  reason  why  many  photographers 
achieve  great  things  in  this  sphere,  and  others  none  at 
all.  As  a  child  can  never  be  long  quiet,  he  must  be 
taken  as  quickly  as  possible;  therefore  such  portraits 
can  only  be  taken  in  fine  weather. 


THE   APPLICATIONS   OF   PHOTOGRAPHY.  153 

The  same  remark  applies  to  groups  with  many 
persons.  No  atelier  has  twenty  or  thirty  props  at  its 
disposal.  Accordingly,  the  photographer  is  frequently 
reduced  to  the  necessity  of  trusting  to  the  good-will  of 
persons  in  sitting  still.  Those  groups  are  very  ugly 
which  show  a  row  of  persons  sitting  beside  each  other 
like  so  many  pagodas.  Photographers  of  superior  refine- 
ment will  occupy  the  attention  of  the  sitter  by  some 
practical  work,  such  as  looking  at  an  album,  eating, 
drinking,  or  card-playing.  In  doing  so,  these  persons 
must  of  necessity  assume  various  positions,  some  show- 
ing their  front  face,  others  their  profile,  and  in  many 
cases  under  circumstances  showing  their  least  favoured 
side.  In  the  case  of  groups,  the  photographer  will 
attend  to  differences  of  complexion,  and  will  find 
difficulties  in  them,  and  in  the  matter  of  dress.  Many 
faces  of  a  dark  complexion  have  too  short  an  exposure 
when  others  have  had  it  long  enough.  But,  as  all  must 
have  the  same  length  of  exposure,  it  is  not  surprising 
that  many  parts  of  the  picture  appear  under  and  others 
over  exposed. 

From  these  causes,  no  one  can  expect  to  appear  to  so 
great  advantage  in  a  group  as  in  a  separate  portrait. 
If  it  happens  so,  this  must  be  ascribed  to  chance. 

It  is  usual  for  people  to  expect  too  much  or  too  little 
from  photographic  groups.  Your  companion  in  the  group 
is  commonly  well  satisfied  if  he  sees  his  own  personality 
given  to  his  fancy,  quite  forgetting  the  ungraceful 
arrangement  of  the  rest,  or  some  ungainliness  among 
his  neighbours,  who  do  not  interest  him  perhaps  so 
much. 

Gentlemen  ought  to  wear  dark  coats.     Light  trousers 


154  THE    CHEMISTRY   OP   LIGHT. 

and  white  waistcoats  often  appear  in  pictures  as  white 
patches,  destroying  the  harmony  of  the  effect,  for  the 
principal  light  ought  not  to  be  concentrated  on  such 
accessories,  but  on  the- head.  Ladies,  in  choosing  their 
toilettes,  generally  overlook  the  abnormal  working  of 
colours.  On  the  occasion  of  the  triumphal  entry  of  the 
Emperor  and  the  German  army  into  Berlin,  in  1871, 
the  young  ladies  chosen  to  grace  the  ceremony  were 
afterwards  photographed  in  their  white  dresses  trimmed 
with  blue,  and  were  not  a  little  surprised  that  the  blue 
trimming  was  as  white  as  the  dress.  Blue  often 
becomes  white  in  photography,  though  there  is  an  excep- 
tion in  the  case  of  the  blue  uniform  of  the  Prussian 
infantry.  On  the  other  hand,  yellow  and  buff  become 
often  black  in  photography  ;  the  same  remark  applies 
to  red.  The  photographer  can  atone  in  some  degree  for 
the  defect  by  a  careful  treatment  of  the  negative  in  the 
case  of  clothes  of  uniform  colour.  The  many-coloured 
toilettes,  however,  now  in  fashion,  produce  a  disastrous 
effect.  Materials  whose  beauty  consists  in  variety  of 
colour,  it  is  evident,  cannot  make  the  same  impression 
in  black  photography  as  in  nature. 

Persons  of  dark  complexion,  also  stout  persons,  should 
prefer  dark  clothes.  It  is  well  known  that  white  clothing 
increases  in  appearance  the  embonpoint  of  the  figure. 
Thin  and  pale  persons  are  advised,  on  the  contrary,  to 
wear  light  clothes,  as  a  pale  complexion  would  appear 
even  paler  when  contrasted  with  black.  Light  clothing 
is  always  to  be  recommended  for  children.  Materials 
should  be  chosen  which  by  their  lustre  make  a  rich  and 
picturesque  impression ;  for  example,  satin,  ribbed  silk, 
taffeta,  and  silky  materials.  Woollen  stuffs  appear  for 


THE    APPLICATIONS    OF    PHOTOGRAPH!.  155 

the  most  part  dull  and  lustreless,  but  they  give  very 
good  effects  in  drapery.  Persons  of  short  and  thick  necks 
would  do  well  to  avoid  high  shirt  collars,  which  make 
the  neck  appear  still  shorter.*  Ladies  with  similar 
attributes  must  lay  aside  velvet,  ribbons,  and  such 
things  around  the  neck;  while  persons  of  long  necks 
will  be  improved  by  such  ornaments. 

The  weather,  the  season,  and  the  time  of  day  present 
serious  difficulties  in  photography. ,  The  days  in  winter 
being  considerably  shorter  and  darker  than  those  in 
summer,  commissions  at  Christmas  are  very  incon- 
venient. Eainy  days  in  winter  are  for  the  most  part 
useless  for  photography;  in  summer  they  are  clear 
enough.  The  hours  immediately  before  and  after  noon 
are  the  most  favourable,  as  we  have  already  stated  in 
our  chapter  on  optics. 

Besides  the  clearness  of  the  weather,  the  amount  of 
light  admitted  by  the  instrument  is  an  important  matter. 
The  lighter  a  picture  appears  produced  by  a  lens,  the 
shorter  may  be  the  time  necessary  for  a  sitting.  A 
lens  increases  strength  in  proportion  to  the  size  of  its 
diameter  and  the  srnallness  of  its  focal  distance.  But 
it  is  by  no  means  possible  to  increase  the  diameter  or 
diminish  the  focal  distance  as  much  as  you  please,  for 
defects  in  the  lenses,  which  have  not  yet  been  overcome, 
stand  in  the  way  of  this.  The  strongest  instruments 
hitherto  constructed  (portrait  lenses)  only  produce  small 
pictures  of  the  size  of  cartes  de  visite,  or  at  most  of 
cabinet  size.  Larger  pictures  can  be  produced  only 
by  weaker  instruments ;  therefore  they  'require  longer 
sittings — a  circumstance  that  makes  the  taking  more 
difficult  in  cloudy  weather  and  with  restless  models  (as 
children)  than  the  preparation  of  smaller  pictures. 


156 


THE    CHEMISTRY    OF   LIGHT. 


Accordingly,  the  latter  show,  on  the  whole,  a  greater 
technical  perfection.  As  they  are  also  very  cheap,  it  is 
natural  that  the  small  cartes  de  visite,  introduced  by 
Disderi  at  Paris,  have  attained  a  general  popularity, 
'and  given  rise  to  a  new  kind  of  album,  displaying  the 
portraits  of  friends  instead  of  poetry,  and  almost  entirely 
supplanting  the  old  scrap-book. 

We  can  only  tolerate  the  modern  album  with  the  out- 
lines of  those  we  love  or  respect  drawn  by  the  light. 


Pig.  67. 

Still  less  can  we  bring  ourselves  to  like  to  see  cartes  de 
visite  suspended ;  they  are  too  small  to  have  any  effect 
from  the  wall,  and  their  frames  are  too  insignificant. 

Photography,  like  engraving,  is  an  art  that  succeeds 
best  on  a  small  scale.  Pictures  of  more  than  quarter 
size  cannot  easily  be  taken  from  nature,  but  life-size 
pictures  are  demanded  by  the  public.  The  photo- 
grapher prepares  these  from  a  small  negative,  by  help  of 


THE    APPLICATIONS   OF   PHOTOGRAPHY.  157 


the   magnifying  apparatus   previously  described  in  the 
chapter  on  optics. 

He  requires  for  these  pictures  the  sun,  which,  un- 
fortunately, in  our  climate  leaves  him  frequently  in  the 
lurch.  The  small  negative  is  placed  in  the  apparatus 
(Fig.  67)  at  N,  and  on  the  table  at  R  a  sheet  of  sensitive 
paper  is  stretched.  Then  the  lens  at  0  projects  a 
magnified  picture  of  the  little  negative  on  the  screen  R, 
and  directly  the  apparatus  is  turned  to  the  sun  the  large 
focal  glass  B  concentrates  sufficient  light  on  the  picture 
to  occasion  a  rapid  browning  of  the  picture,  and  under 
favourable  circumstances  a  life-size  copy  can  be  taken 
in  fifteen  minutes. 

Much  has  been  said  of  instantaneous  pictures.  The 
Deputy  Faucher  remarked  once,  in  the  Prussian  House 
of  Deputies,  "  There  are  now  instantaneous  pictures. 
Portraits  can  be  stolen  by  this  process,  and  it  will  per- 
haps be  necessary  to  guard  against  it  by  the  most 
extraordinary  precautions — probably  masks  will  have  to 
be  worn."  This  statement  is  based  on,  a  mystification. 
Faucher  had  been  made  the  victim  of  one  of  those 
photographers  who,  by  incredible  boasting  and  by  puff- 
ing themselves,  seek  to  impose  on  the  public.  In- 
stantaneous pictures  are  possible  if  the  object  is  clearly 
illumined  by  the  sun ;  therefore  it  is  easier  to  prepare  an 
instantaneous  picture  from  a  brightly  illumined  land- 
scape. It  is  quite  another  matter  having  to  do  with  a 
portrait  in  an  atelier.  Direct  sunlight  would  produce 
an  unpleasant  glare  and  sharp  shadows  upon  a  dark 
background  in  a  portrait  under  these  circumstances: 
the  eyes  would  have  a  contracted  look,  and  a  very  ugly 
picture  would  be  the  result.  As  we  before  remarked, 


158  THE    CHEMISTRY   OF   LIGHT. 

very  powerful  lenses  have  been  constructed,  which  admit 
of  shortening  the  time  of  exposure.  These  are,  however, 
only  suitable  for  very  small  pictures,  and  are  only 
employed  for  small,  restless  objects,  like  children,  in 
whose  case  the  photographer  is  satisfied  to  get  the  chief 
part — that  is,  the  head — as  quickly  as  possible  into  the 
picture. 

It  occurs  frequently  that  persons  desire  a  fresh 
impression  from  a  former  photographic  picture.  We 
here  remark  that  such  an  impression  may  be  good,  but 
that  a  photograph  taken  from  a  photograph  is  never  so 
fine  as  an  original  picture.  The  cause  of  this  is,  on  the 
one  hand,  the  brown  tone  of  the  photograph,  which 
possesses  very  little  activity  photographically;  whilst, 
on  the  other  hand,  the  paper  beneath  has  an  undue 
effect.  This  is  sometimes  glazed,  and  then  produces  a 
false  light  upon  the  reproduced  picture ;  or  it  is  rough, 
and  then  the  veins  of  the  paper  at  once  cast  a  shade, 
which  gives  to  the  picture  a  disagreeable,  coarse-grained 
appearance.  On  that  account  it  is  easy,  even  for 
unpractised  eyes,  to  distinguish  the  copy  from  the 
original  photograph.  Such  copies  are  frequently  to  be 
met  with  at  fairs  and  in  stationers'  shops,  and  can 
be  bought  for  a  ridiculously  small  price.  In  most 
countries,  however,  copying  from  original  photographs 
is  forbidden  as  piracy,  and  this  prohibition  ought  soon 
to  be  introduced  into  Germany. 

It  has,  indeed,  been  remarked  that  piracy  is  advan- 
tageous to  the  public  by  making  favourite  pictures  at  a 
low  price.*  But  this  advantage  is  no  compensation  for 

*  Precisely  the  same  argument  can  bo  advanced  to  defend  the  piracy 
of  books,  which  is  forbidden. 


THE   APPLICATIONS   OF   PHOTOGEAPHY.  159 

the  injury  thus  inflicted  on  the  author  of  the  original 
photographs — he  often  incurring  considerable  expense 
in  taking  photographs  in  the  Hartz  Mountains,  and  the 
Thiiringer  Wald,  or  in  making  an  imposing  picture  of 
a  distinguished  person.  A  great  undertaking  of  this 
kind  is  seldom  successfulat  first,  and  if  his  production 
is  not  protected  by  the  law,  he  will  prefer  to  give  up 
producing  such  original  pictures. 

SECTION  II. — LANDSCAPE  PHOTOGRAPHY, 

Its  Scope — Difficulty  of  Taking  Landscapes — The  Photographic  Tent — 
Significance  of  Landscape  Photography  applied  to  Geography — The 
Dry  Plates — Stereoscopic  Landscapes — Transparent  Stereoscopes — 
Panoramic  Pictures. 

Landscape  photography  is  a  branch  much  less  pur- 
sued than  portrait  photography.  While  portrait  photo- 
graphy is  mostly  carried  out  on  direct  application,  it 
is  a  very  rare  thing  to  receive  orders  for  a  landscape. 
These  views  are  left  to  speculators,  who  employ  photo- 
graphy as  a  means  of  representing  favourite  localities 
in  largely  frequented  countries,  and  thereby  making  a 
profit  with  tourists.  Thus  photographers  wander 
through  the  noteworthy  sites  of  our  capitals  and  moun- 
tains, and,  as  the  originals  are  accessible  to  every  one, 
each  of  the  competitors  tries  to  outdo  his  fellows  by 
excellence  of  work  or  cheapness.  The  reproducer  is 
associated  with  these  original  photographers ;  he  does  not 
undertake  any  costly  journeys,  but  awaits  the  issue  of 
original  photographs  to  copy  them  at  once,  and  offer 
them  at  a  low  price.  The  inclination  of  the  public  is  here 
favourable  to  the  cheap  seller.  A  landscape  is  seldom 
bought  for  its  value  as  a  work  of  art,  but  more  as  a 


160  THE    CHEMISTRY   OF   LIGHT. 

souvenir  of  a  happy  hour,  or  as  a  reminiscence  which  in 
subsequent  years  will  recall  some  interesting  object, 
whether  a  statue  or  a  castle ;  therefore  less  is  demanded 
in  the  case  of  landscape  and  architecture  Views,  and 
this  is  the  reason  why  landscape  photography  is  not  at 
present  in  a  very  high  state  of  perfection.  The  English 
are  relatively  the  best  in  this  branch,  because  they  ask 
good  prices  for  their  pictures  and  are  protected  against 
piracy.  The  Swiss  views  of  Mr.  England  have  a 
universal  celebrity  in  Germany ;  pictures  of  equal  merit 
are  only  produced  by  Baldi  and  Wiirthle  at  Salzburg. 
Braun  of  Dornach  also  deserves  an  honourable  men- 
tion, having  produced  excellent  landscapes,  his  Swiss 
views  being  known  everywhere. 

Superficial  observers  entertain  the  belief  that  landscape 
photographs  must  be  as  good  as  others,  as  the  object 
remains  always  the  same,  and  all  are  prepared  by  the 
same  process. 

Both  assumptions  are,  however,  erroneous.  The  object 
is  not  always  the  same,  for  a  landscape  appears  under 
very  different  aspects  in  the  morning  and  evening  light, 
or  in  fine  and  clear  weather.  Whoever  studies  these 
effects  of  light  will  soon  discover  at  what  hour  a  land- 
scape will  look  most  beautiful,  and  will  choose  it  for 
taking  his  view.  Accordingly,  his  picture  will  surpass 
greatly  that  of  a  superficial  and  hasty  photographer,  who 
takes  the  landscape  as  he  finds  it.  The  choice  of  the 
position  is  equally  important.  By  comparing  pictures 
taken  at  different  heights  (see  page  147),  it  is  evident 
that  the  whole  scene  in  many  landscapes  changes  by 
standing  a  little  higher  or  lower,  or  a  few  more  paces  to 
the  right  or  to  the  left.  The  man  having  the  eye  of  an 


THE   APPLICATIONS   OF   PHOTOGEAPHY.  161 

artist,  who  knows  how  to  seek  the  best  position,  will  at 
all  times  give  the  best  picture. 

The  same  remark  applies  to  taking  architecture  and 
sculpture.  It  is  evident  that  a  photographer  who  under- 
takes this  must  be  partially  favoured  by  wind  and 
weather.  A  breeze  stirring  the  trees  often  injures  his 
view,  which  may  be  impeded  the  whole  day  by  wind  and 
rain.  To  this  difficulty  may  be  also  allied  that  of  an 
unmannerly  class  of  people,  who  insist  on  being  taken 
with  the  picture,  and  thrust  themselves  full  in  front  of 
the  photographic  apparatus,  making  the  attempt  im- 
possible— a  weakness  that  is  more  commonly  met  with 
in  Germany  than  elsewhere,  and  is  the  more  inexplicable 
as  they  generally  never  see  anything  more  of  the 
picture. 

It  is  a  great  inconvenience  for  the  landscape  photo- 
grapher that  all  the  chemicals,  cups,  bottles,  glasses, 
which  are  requisite  for  the  process,  must  be  carried  on 
the  journey ;  nay,  more,  the  photographer  needs  a  trans- 
portable dark  chamber  in,  which  he  can  prepare  his 
sensitive  plates. 

The  accompanying  figure  represents  an  apparatus  of 
this  kind,  together  with  the  photographer.  It  is  only 
the  upper  part  of  his  body  that  is  in  the  tent,  but  the 
intermediate  space  is  impervious  to  light  through  the 
curtain.  For  the  sake  of  facility  of  transport,  everything 
in  a  dark  tent  of  this  kind  is  contracted  into  the  smallest 
space.  A  yellow  glass  q  lets  light  into  the  interior. 
The  silver  bath  is  in  a  box  y,  and  the  necessary  water 
is  in  the  cistern  x,  from  which  a  pipe  -passes  into  the 
interior.  The  whole  tent  can  be  folded  up,  and  forms  a 
box  of  the  size  of  z  in  the  figure. 


162 


THE    CHEMISTKY   OF   LIGHT. 


Though  these  arrangements  are  very  compact  and 
compendious,  they  are  still  of  considerable  weight, 
making  the  ascent  of  difficult  places,  such  as  the 
Finsteraarhorn,  the  Wetterhorn,  and  the  Jungfrau,  im- 
possible. 


Fig.  68. 

Dry  plates  are  important  to  produce  views  of  this 
kind,  as  they  can  be  prepared  at  home  and  taken  on  a 
journey.  They  dispense  with  the  dark  tent,  collodion, 
silver  bath,  and  the  water  for  washing.  The  dry  plate 
and  photographic  apparatus  suffice  in  this  case.  We 
have  already  described  the  production  of  dry  plates,  and 
how  they  are  prepared  by  washing  an  ordinary  plate  of 


THE    APPLICATIONS    OF    PHOTOGRAPHY.  163 

sensitized  collodion,  pouring  on  it  some  substance 
absorbing  iodine,  such  as  tannin,  and  then  leaving  it  to 
dry.  Unfortunately,  plates  prepared  in  this  manner  are 
less  sensitive  than  the  fresh  wet  plates,  and  the  pictures 
they  give  appear  less  fine  than  those  taken  with  the 
freshly  prepared  plates ;  the  results,  therefore,  cannot  be 
depended  upon.  Many  plates  spoil  after  a  lapse  of 
time ;  then  the  view  obtained  cannot  be  judgel  till  it  has 
been  touched  up  at  home,  and  the  result  is  often  very 
imperfect,  nor  can  it  be  corrected  far  from  the  place 
where  it  was  taken.  For  these  reasons  the  moist  process 
has  been  preferred  in  taking  landscapes,  notwithstanding 
its  inconvenience,  and  only  a  few  photographers  work 
with  dry  plates. 

Stereoscopic  views  are  very  popular  in  the  landscape 
branch.  Though  so  limited  in  size,  they  present  land- 
scapes in  so  plastic  a  form  that  they  distance  even 
larger  pictures  in  their  effect.  We  have  already  described 
how  these  views  are  taken.  If  the  light  is  bright  and 
the  lens  large,  instantaneous  views  can  be  taken  with 
the  stereoscopic  apparatus,  and  have  been  offered  largely 
for  sale. 

These  transparent  stereoscopic  views  on  glass,  pre- 
pared by  Ferrier  and  Soulier,  are  wonderfully  beautiful. 
They  are  produced  on  a  collodion  film,  by  placing  the 
glass  negative  taken  from  nature  on  a  dry  plate,  and  then 
exposing  it.  It  then  copies  the  negative  exactly  in  the 
same  manner  upon  the  sensitized  glass  plate,  or  upon 
the  sensitized  paper,  only  the  invisible  light  impression 
must  be  first  developed  by  the  application  of  pyrogallic 
acid.  As  the  production  of  such  glass  positives  requires 
a  more  minute  and  lengthened  treatment  than  the  paper 
pictures,  their  price  is  higher. 


164  THE    CHEMISTRY   OF   LIGHT. 

Latterly,  however,  by  the  help  of  a  printing  process 
(the  Woodbury  process),  it  has  become  possible  to  pro- 
duce these  glass  pictures  at  a  considerably  cheaper  rate. 
We  refer  to  this  process  further  on. 

Though  at  first  sight  landscape  photography  may 
appear  unimportant,  yet  it  is  of  the  greatest  use  for 
geographical  information.  There  is  no  better  medium 
for  conveying  a  true  picture  of  foreign  lands,  of  rocks, 
plants,  and  animal  forms,  than  photography.  It  has 
even  become  an  essential  auxiliary  in  exploring  expedi- 
tions, being  alone  capable  of  giving  a  perfectly  faithful 
description  of  what  has  been  seen.  I  admit  that  the 
inconvenience  of  transporting  a  photographic  apparatus, 
and  the  injury  to  which  the  chemicals  are  easily  exposed, 
limit  the  use  of  photography  in  exploring  expeditions, 
and  require  a  very  expert  photographer ;  but  that  these 
obstacles  can  be  overcome  is  proved,  among  others,  by 
the  excellent  views  taken  by  Count  Wilzek  and  Burger 
at  Nova-Zembla,  Baron  Stillfried  in  Japan,  Burger  and 
Lyons  in  India,  and  Dr.  G.  Fritzsch  in  South  Africa.  We 
shall  show  in  the  following  chapter  the  importance  of 
landscape  photographs  for  land  surveying. 

Panoramic  views  are  quite  a  special  branch  of  land- 
scape photography.  The  noted  photographer  Braun,  oi 
Dornach  (Alsace),  offered  for  many  years  pictures  for 
sale  which  contained  half  the  circumference  of  the 
panoramas  of  the  Bigi,  of  the  Faulhorn,  of  Pilatus,  and 
other  well-known  points.  It  is  evidently  impossible  for 
a  fixed  camera  to  command  at  once  such  a  panorama  ; 
nor  can  the  human  eye  do  this,  for  the  most  we  can 
survey  is  90°,  and  this  is  only  a  quarter  of  a  circum- 
ference. If  we  wish  to  see  the  whole  circumference,  we 


THE    APPLICATIONS    OF    PHOTOGRAPHY.  165 

are  obliged  to  turn  round.  Martens,  a  German  engraver 
residing  at  Paris,  conceived  the  idea  of  taking  pano- 
ramic views  with  the  help  of  a  revolving  camera,  or  of 
a  revolving  lens  in  a  camera.  Let  the  reader  imagine 
a  camera  with  a  cylindrical  hinder  surface  p  p  (Fig.  69) 


Fig.  69. 


represented  in  outline,  also  a  lens  o.  t  Then  the  image 
of  any  point  a  is  situated  on  the  line  a  o  b,  which  is 
drawn  from  a  through  the  centre  of  the  objective.  If 
the  lens  revolves  round  its  centre,  the  image  remains 


166  THE    CHEMISTRY   OF   LIGHT. 

immovably  at  its  place  I ;  if  it  were  to  revolve  to  any 
other  point  than  its  centre,  the  image  would  be  dis- 
placed. It  is  therefore  evident  that,  if  the  lens  revolves 
round  its  centre,  it  can  imprint  successively  an  image  of 
half  the  horizon  on  the  cylindrical  surface.  The  only 
problem  then,  therefore,  is  to  produce  a  sensitive  cylin- 
drical surface.  This  is  not  difficult  with  sensitive  paper, 
but  much  more  difficult  with  glass,  which  is  extremely 
brittle  in  this  form.  Accordingly,  Brandon  introduced  a 
smooth  plate,  which  rolls  as  it  were  round  the  cylinder 
p  p ;  that  is,  which  during  the  revolution  of  the  lens  is 
moved  in  such  a  fashion  that  it  always  remains  perpen- 
dicular to  the  axis  o  b  of  the  lens.  The  mechanism  of 
such  a  camera  is  rather  complicated,  but  it  has  main- 
tained its  ground  in  practice,  and  numerous  panoramic 
views  have  been  taken  with  it.  We  must  confine  our- 
selves here  to  cursory  remarks ;  those  who  seek  for  further 
details  are  referred  to  Vogel's  "  Manual  of  Photography." 

SECTION  III. — PHOTO  GRAMMETRY,  OR  LEVELLING  BY  PHOTOGRAPHY. 

Relation  between  Photography  and  Measurement — Principle  of  Trigono- 
metrical Measurement — Projection  of  Maps — Photographic  Measure, 
nient  of  Altitudes. 

An  essential  difference  between  a  photographic  view 
and  an  artist's  painting  is  the  fact  of  its  not  being  the 
production  of  the  operator's  will,  but  that  its  outline  and 
design  are  subject  to  determinate  laws.  All  photo- 
graphic views  are  produced  by  means  of  lenses.  A  lens 
view  of  this  kind  is  always  an  exact  central  perspective  ; 
that  is,  each  point  of  view  lies  on  the  straight  line 
which  can  be  drawn  through  the  optical  centre  of  the 
lens.  Let  a  b  c  (Fig  70)  be  three  objects  in  nature,  K 


THE   APPLICATIONS   OF   PHOTOGRAPHY.  167 

a  camera  (of  which  the  outline  is  given  to  facilitate  com- 
prehension), and  I  its  lens.  Then  the  images  of  the 
different  objects  are  situated  on  the  produced  short  lines 
a  o,  b  o,  c  o — that  is  to  say,  on  a'  V  d ;  therefore  they  have 
in  the  picture  exactly  the  same  relative  position  as  in 
nature.  Accordingly,  a  good  photograph  can  serve  to 
determine  accurately  the  position  of  objects  in  nature  ; 
that  is,  to  construct  maps  of  the  piece  of  ground  that 
has  been  taken  in  the  view. 

For  example,  let  the  reader  conceive  an  image,  which 
stands  upright  in  the  camera  of  the  diagram  annexed, 
brought  down  flat  upon  the  paper.     Then  in  the  middle 
of  the  field  of  view,  at  the  tree  &',  let  a  perpendicular 
line  be  drawn  equal  to  the  focal  distance  o  b '.     It  is  only 
necessary,  after  this,  following  the  figure,  to  construct 
the  lines  d  o  and  a  o  Ff  o,  in  order  at  once  to  find  the 
directions  in  which  the  tower,  the  flag,  and  the  trees 
will  be  seen  from  the  position  P.    If  now  a  second  view 
be  taken  from  a  point  P'  which  lies  in  the  direction  of 
the  flag  F,  a  second  view  is  obtained  c"  V  a",  which 
naturally  looks   quite  different  from  the  first  in  con- 
sequence of  the  change  of  position.     If  this  view  from 
the   second  position  be   also  brought  down  to  it,  and 
a  line  6"  o,  equal   in   length  to  the  focal  distance,  be 
drawn  to  the  second  position,  then  .the  lines  c"  o  and  a,"  o 
give  again  the  direction  of   the  lines  from  a  b  c.      If 
these  lines  be  sufficiently  produced  on  the  paper,  they 
intersect  at  points  the  situation  of  which  corresponds 
exactly  to  that  of  the  object ;  and  thus,  in  two  views  at 
two  points,  the  means  is  afforded  of  constructing  a  map 
in  which  the  situation  of  all  points  contained  in  both 
views  is  exactly  given. 


168 


THE    CHEMISTRY    OF    LIGHT. 


A  different  procedure  is  followed  in  ordinary  trigo- 
nometry. In  that  science,  the  first  step  would  be  to 
measure  the  distance  P  P',  then  to  set  up  an  instrument 
for  taking  angles  at  P,  and  to  determine  the  angle  made 
by  the  line  P  Pr  with  the  lines  a  o,  b  o,  c  o ;  the  same 


operation  being  repeated  at  the  other  end  of  what  is 
called  the  station  line  P  P'.  It  is  evident  that  as  many 
measurements  must  be  made  at  both  points  as  there 
are  objects  of  interest,  whereas  a  photographic  view 


THE    APPLICATIONS   OF    PHOTOGKAPHY.  169 

taken  once  for  all  fixes  all  objects  correctly  in  their 
relative  positions.  Accordingly,  there  is  a  considerable 
economy  of  time  in  applying  photography ;  and  this  is  of 
great  moment  in  war,  when  frequently,  in  consequence 
of  interruptions  on  the  part  of  the  enemy,  the  leisure  is 
wanted  which  is  necessary  for  triangulation ;  also  in 
journeys,  when  the  stay  at  particular  places  is  too  short 
to  make  observations  requiring  time. 

Therefore  this  process  has  great  advantages  in  explor- 
ing expeditions;  and  landscapes  taken  photographically 
have  a  twofold  value  :  not  only  do  they  give  a  view  of 
the  country,  but  also  data  for  the  projection  of  maps.  I 
admit  that  to  this  end  two  views  are  necessary,  which 
must  be  taken  from  the  end  of  a  station  line.  Then  the 
taking  of  these  views  must  be  carried  out  with  mathe- 
matical accuracy :  the  camera  must  be  placed  in  a 
perfectly  horizontal  position ;  its  lens  must  give  a  per- 
fectly correct  image  ;  the  plates  must  be  absolutely  level, 
etc.  But  all  these  conditions  are  not  easily  obtained.  To 
this  other  difficulties  are  added,  proceeding  from  the  very 
nature  of  photography.  This  art  requires  clear,  bright 
weather ;  with  a  troubled  sky,  or  when  the  atmosphere  is 
veiled — the  aerial  perspective  of  landscape  painters — it 
often  gives  remote  objects  GO  indistinctly  in  the  view 
that  no  correct  measurement  can  be  made  of  them,  though 
the  surveyor  can  clearly  distinguish  and  measure  from 
nature  in  such  weather.  Further,  the  direct  action 
of  the  sun  offers  difficulties  to  photography.  If  it 
stands  in  front  of  the  camera — that  is,  if  it  shines  full  on 
the  objective — it  often  occasions  fogging  on  the  plate, 
greatly  modifying  the  value  of  the  view  for  purposes  of 
measurement.  All  these  circumstances  militate  against 


170 


THE    CHEMISTRY   OF   LIGHT. 


the  application  of  photogrammetry,  as  this  mode  of 
measurement  has  been  called  by  Meydenbauer,  who  has 
long  used  it.  Meydenbauer  prepared  a  good  map  of  the 


Unstrutthal  according  to  this  method.  But  the  experi- 
ences during  the  campaign  of  1870  are  not  so  satis- 
factory (the  royal  Prussian  staff  tried  the  process 


THE   APPLICATIONS   OF    PHOTOGRAPHY.  171 

before  Strasburg) — perhaps  the  imperfection  of  the 
apparatus  occasioned  the  unsatisfactory  results.  It  is 
to  be  hoped  that  future  attempts  will  succeed  in  making 
this  important  method  practically  available  in  the  in- 
terests of  geography. 

Photography  can  determine  the  elevation  of  moun- 
tains and  of  buildings,  as  well  as  determine  positions  in 
a  plain.  Let  it  be  supposed  that  a  b  (Fig.  71)  is  a  tower, 
represented  in  the  accompanying  diagram  as  imprint- 
ing the  image  a  bf  on  the  photographic  apparatus.  It  is 
evident  that  the  image  will  be  smaller  than  the  object. 
According  to  a  well-known  mathematical  principle,  the 
magnitude  of  the  image  a!  b'  to  that  of  the  tower  a  &  is 
as  the  distance  of  the  image  from  the  objective  o  r  to  the 
distance  of  the  tower  from  the  objective.  This  gives  the 
proportion : 

o  r  :  E  =  a  U :  x, 

in  which  by  E  is  understood  the  distance  of  the  tower 
from  the  camera,  which  can  be  measured.  The  height 
of  the  tower  can  be  easily  found  from  the  above  pro- 
portion. 

Meydenbauer  has  deduced  the  dimensions  of  the 
ground-plan  and  elevation  of  a  house  from  its  photo- 
graph. 

SECTION  IY. — ASTRONOMICAL  PHOTOGRAPHY. 

Its  Application — The  Photographic  Telescope — Taking  Eclipses— Pro- 
tuberances — Corona — Sun-spots— Enlarged  Images  of  the  Sun — 
Eutherford's  Labours— Astral. Photography — Pictures  of  the  Moon 
— Spectral  Photography — Photography  and  the  Transit  of  Venus. 

The  province  of  astronomical  photography  may  embrace 
two  kinds  of  operations :  first,  it  has  to  give  a  faithful 


172  THE    CHEMISTRY    OF    LIGHT. 

representation  of  the  phenomena  of  the  heavens — which 
change  so  rapidly  that  the  operator  cannot  follow  them ; 
for  example,  the  phenomena  of  eclipses,  or  others  which 
are  inconvenient  to  represent,  such  as  sun-spots. 
Secondly,  astronomical  photography  has  to  produce 
views  of  heavenly  bodies  that  can  be  used  for  measure- 
ments. Photography  has  made  successful  attempts  in 
both  these  walks,  and  it  is  employed  daily  as  an 
auxiliary  to  produce  views  of  sun-spots  at  several  obser- 
vatories ;  for  instance,  in  Germany,  at  the  observatory 
of  Herr  Von  Biilow,  Privy  Councillor,  at  Bothkamp, 
near  Kiel. 

The  art  and  mode  of  preparing  astronomical  pictures 
differs  little  from  that  of  ordinary  photographs.  An 
ordinary  photographic  apparatus  can  be  used  for  this 


Fig.  72. 


purpose,  were  it  not  that  it  gives  too  minute  views 
of  very  remote  objects,  as  the  stars.  The  size  of  the 
picture  bears  a  direct  relation  to  the  focal  distance  of 
the  lens ;  therefore,  in  taking  astronomic  photographs, 
astronomic  lenses  are  used  where  focal  distance  is  very 
long,  by  converting  an  astronomic  telescope  into  a 
photographic  instrument. 

The  accompanying  figure  shows  a  telescope  of  this 
kind  adapted  to  photographic  purposes.  The  objective 
O  remains  in  its  place,  the  eye-piece,  which  is  fixed 


THE   APPLICATIONS   OF 


at  the  other  end  of  the  tube,  is  arr^ftwaj,'^aji  an 
apparatus  V  (Fig.  72)  is  substituted  for  it,  which  is 
identical  with  the  hinder  part  of  a  photographic  camera 


Fig.  73. 


Thus  it  contains  a  ground  glass  slide  S,  which,  after  this 
apparatus  has  been  firmly  fitted  in,  can  be  exchanged 


174  THE   CHEMISTRY   OF   LIGHT. 

for  a  sensitive  plate.  This  firm  fitting  is  effected  by 
moving  J;he  spring  T. 

But  now  a  difficulty  occurs  in  the  movement  of  stars, 
which  leads  to  the  necessity  of  the  telescope  following 
this  movement  if  the  views  are  to  be  sharply  brought 
out.  To  this  end  the  stand  of  the  telescope  is  furnished 
with  a  clockwork  p  p  that  causes  it  to  revolve  in  the 
direction  of  the  course  of  the  stars,  so  that  the  telescope 
is  what  is  called  parallactically  set  up.  Fig.  73  shows 
an  arrangement  of  this  kind. 

The  oblique  leg  of  the  telescope  resting  on  the  foot  a 
is  parallel  to  the  earth's  axis.  On  this  stand  the  polar 
axis  of  the  telescope  revolves  with  the  hour  circle  fi,  by 
the  working  of  the  clockwork,  once  every  twenty-four 
hours. 

The  telescope  d  d  is  not  fixed  immediately  on  the 
polar  axis,  but  on  an  axis  c,  perpendicular  to  it ;  it  can 
be  turned  round  the  latter  (the  declination  axis)  in  all 
directions  perpendicular  to  the  axis  c  i.  It  is  only  the 
movement  of  the  two  axes  that  allows  any  star  you 
please  to  be  brought  into  the  field  of  view  of  the  tele- 
scope. 

The  first  attempt  to  employ  photography  for  astro- 
nomic views  was  made  by  Berkowsky,  at  the  Eoyal 
Observatory,  in  the  year  1851,  by  the  help  of  Bessel's 
noted  heliometer,  during  a  total  eclipse  of  the  sun.  He 
obtained  a  dageurreotype,  the  beauty  of  which  was  much 
lauded,  and  which  showed  very  well  the  remarkable  phe- 
nomena that  appear  during  an  eclipse  of  the  sun — flame- 
like  formations  that  stand  out  in  the  darkened  disk  of 
the  sun,  being  what  are  called  protuberances.  In  the 
year  1860,  Warren  de  la  Eue  in  England,  and  Secchi  at 


THE   APPLICATIONS   OF   PHOTOGEAPHY.  175 

Eome,  undertook  an  expedition  to  Eivabellosa,  in  Spain, 
to  observe  the  eclipse  of  the  sun,  and  both  produced 
interesting  views  on  collodion  plates.  In  1868,  the 
Government  of  the  North  German  Confederation 
equipped  an  expedition  to  observe  the  eclipse  of  August 
18th,  and  sent  Dr.  Fritzsch,  Messrs.  Zencker,  Tiele,  and 
the  author  of  this  work  to  take  photographs.  Another 
photographic  expedition  was  sent  out  by  the  English 
Government  to  India.  Besides  these,  the  German, 
English,  Austrian,  and  French  Governments  sent  out 
expeditions  for  the  ocular  observation  of  the  phenomenon. 

Obstacles  were,  no  doubt,  encountered  by  these  ex- 
peditions, nevertheless  they  produced  results  that  finally 
settled  the  question  about  the  nature  of  the  protube- 
rances, and  moreover  gained  experience  that  materially 
lessened  the  labour  of  subsequent  photographic  observers. 

We  proceed  to  introduce  a  description  of  the  expe- 
dition to  Aden,  giving  a  faithful  account  of  the  obstacles 
associated  with  an  undertaking  so  simple  in  appearance. 
The  author  wrote  from  Aden  the  following  account  of 
his  arrival  and  residence  at  that  place  : — 

"  The  aspect  of  Aden  is  by  no  means  cheerful.  An 
almost  entirely  bald,  savage,  and  broken  mass  of  rock, 
the  remains  of  an  extinct  volcano;  in  front  and  in  the 
midst  warehouses,  shops,  coal-sheds,  flag-staffs,  etc. ; — 
such  was  the  appearance  of  the  place  that  was  to  be 
our  residence  for  a  fortnight.  The  colour  green  was 
entirely  wanting  in  nature. 

"Our  luggage  and  ourselves  wrere  conveyed  to  land 
amid  the  shouts,  quarrelling,  and  tumuft  of  the  Arab 
mob.  On  landing  we  learned  that  our  colleagues,  who 
had  preceded  us,  had  been  received  in  the  most  cour- 


176  THE    CHEMISTRY    OF   LIGHT. 

teous  manner  by  the  British  authorities,  and  that  two 
Indian  huts — bungalows,  usual  in  this  climate — on  the 
east  side  of  the  peninsula,  had  been  assigned  us  as  our 
station. 

"After  a  long  search  we  found  the  locality  and  our 
comrades,  together  with  the  members  of  the  Austrian 
expedition,  Dr.  Weiss  and  Messrs.  Oppolzer  and  Eiha,  and 
in  as  excellent  quarters  as  could  be  wished  on  this  desert 
coast.  The  English  authorities  acted  the  part  of  host 
in  the  most  generous  manner.  A  whole  staff  of  servants, 
cook,  etc.,  waited  upon  us;  carriages,  camels,  and 
donkeys  were  at  our  disposal,  and  all  our  wishes  were 
gratified  or  anticipated.  Thus  our  bodily  comfort  had 
little  to  desire ;  the  temperature  (26°  Eeaumur),  might 
be  called  low  for  the  Eed  Sea,  for  a  fresh  breeze  was 
always  blowing  on  the  heights  of  the  Marshagill,  on 
which  our  bungalows  stood,  and  contributed  greatly  in 
refreshing  the  air. 

"  There  still  remained  ten  days  to  prepare  for  taking 
views  of  the  eclipse.  They  were  employed  in  preparing 
stands  for  our  telescopes,  in  putting  them  up,  and  in 
setting  them  in  order.  We  used  as  observatory  a 
bungalow,  which  we  partly  unroofed,  in  order  to  look 
through  the  roof  with  the  telescope,  and  we  converted 
the  rest  of  its  interior  into  a  laboratory,  wTashing-room, 
and  store-room.  In  this  telescope  cage — for  it  was 
nothing  more — we  were  tolerably  protected  from  the 
wind,  but  less  so  from  the  dust.  Water  was  brought  to 
us  by  donkeys,  in  leather  bags.  Two  tents  that  we  had 
brought  from  Europe  answered  the  purpose  of  dark 
chambers.  Spare  apparatus  for  taking  landscapes  and 
portraits,  that  we  had  brought  with  us,  gave  us  the 


THE   APPLICATIONS   OF   PHOTOGRAPHY.  177 

material  for  taking  views  of  the  country  and  its  popula- 
tion, and  were  also  a  useful  means  of  testing  our 
chemicals. 

"  Some  trifling  defects  in  the  latter  were  quickly 
remedied,  but  it  was  not  so  easy  to  remove  the  effects 
of  the  dust  and  human  exhalations.  During  the 
slightest  exertion  in  that  damp  atmosphere,  the  per- 
spiration flowed  from  the  body  in  streams :  it  ran  from 
the  tips  of  the  fingers,  dropped  from  the  face,  and  often 
a  well-cleansed  or  prepared  plate  was  spoilt  by  a  drop 
of  sweat.  Nevertheless,  practice  taught  us  how  to 
encounter  this  obstacle;  some  attempts  at  taking  the 
sun,  etc.,  turned  out  successful;  we  were  able  to  look 
on  tranquilly  to  the  eclipse.  Only  one  thing  gave  us 
serious  uneasiness, — that  was  the  weather.  All  accounts 
of  Aden  had  unanimously  represented  its  sky  as  perfectly 
clear,  competent  witnesses  having  asseverated  that  it 
rained  there  at  most  three  times  a  year,  and  that 
clouds  were  exceptions. 

"We  were  therefore  not  a  little  surprised  when,  on 
our  arrival,  we  found  the  volcanic  heights  of  Aden  quite 
concealed  with  clouds,  and  when  we  were  greeted  with  a 
shower  of  rain  on  the  following  morning.  But  we 
became  still  more  anxious  when,  day  after  day,  the  sun 
was  concealed  by  clouds,  and  this  weather  became  worse 
rather  than  better  in  the  course  of  time.  The  prospect 
of  succeeding  in  our  main  object  looked  dreary  enough, 
and  soon  all  our  hopes  deserted  us. 

"  On  the  day  of  the  eclipse  we  left  our  quarters  about 
4  a.m.  Nine-tenths  of  the  sky  were  clotfdy.  We  set  to 
wrork  in  a  resigned  frame  of  mind.  It  was  the  under- 
taking of  the  North  German  expedition  to  photograph 


178  THE    CHEMISTRY   OF   LIGHT. 

the  eclipse  throughout  its  continuance.  For  this  pur- 
pose we  used  a  telescope  with  a  six  inch  lens  without 
focal  difference,  with  a  focal  distance  of  six  feet.  This 
lens,  made  by  Steinheil,  gave  an  image  of  the  sun  three- 
fourths  of  an  inch  in  diameter,  which  could  be  taken  on 
a  photographic  plate  by  the  help  of  an  ordinary  box 
with  slides  for  two  views.  As  the  sun  and  moon  move, 
such  an  instrument,  if  stationary,  would  only  give  ill- 
defined  views.  Accordingly,  the  telescope  was  connected 
with  wheelworks  that  gave  it  a  movement  correspond- 
ing with  that  of  the  heavenly  bodies.  To  avoid  all 
agitation  of  the  telescope,  the  closing  lid  of  the  objective 
was  not  placed  close  to  the  telescope,  but  at  a  separate 
stand,  and  was  connected  with  the  telescope  by  an 
elastic  hood. 

"  The  duration  of  the  total  eclipse  was  at  Aden  three 
minutes,  in  India  five.  We  had,  however,  chosen  our 
station  at  Aden  because  photographic  observers  were 
already  present  in  India,  and  because  the  eclipse  began 
first  at  Aden  (about  an  hour  sooner  than  in  India).  Thus, 
by  comparing  our  observations  with  those  in  India,  a 
judgment  might  be  formed  whether  those  wonderful 
phenomena  of  light  called  protuberances,  during  a  total 
eclipse,  changed  or  did  not  change  in  the  course  of  the 
eclipse.  It  was  our  present  endeavour  to  take  as  many 
views  as  possible  of  the  phenomenon  in  three  minutes. 
To  this  end  we  had  regularly  practised,  as  artillerymen 
do  with  their  cannon. 

"  Dr.  Fritzsch  prepared  the  plates  in  the  first  tent, 
Dr.  Zencker  pushed  the  boxes  into  the  telescope, 
Dr.  Tiele  exposed  them  and  developed  them  in  the 
second  tent. 


THE   APPLICATIONS   OF   PHOTOGEAPHY.  179 

"We  had  determined  that  it  was  possible  in  this 
manner  to  take  six  views  in  three  minutes. 

"  The  decisive  moment  approached.  The  cloudy  sky, 
anxiously  surveyed  by  us,  showed  to  our  great  satisfac- 
tion a  few  breaks  through  which  the  disc  of  the  sun, 
partly  concealed  by  the  moon,  and  appearing  as  a 
crescent,  was  visible.  The  landscape  appeared  in  the 
strangest  light,  being  almost  a  half-and-half  mixture 
between  sunlight  and  moonlight.  The  chemical  influ- 
ence of  light  showed  itself  very  weak.  An  experimental 
plate  only  gave  a  view  of  the  clouds  after  fifteen  seconds' 
exposure.  The  sun's  crescent  became  gradually  smaller, 
the  break  in  the  clouds  gradually  increased,  and  we  took 
heart. 

"The  last  minutes  preceding  the  total  eclipse,  which 
occurred  at  6.20,  fled  on  wings.  Dr.  Fritzsch  and  I  crept 
hastily  into  our  tent  and  remained  there,  preparing 
plates  and  developing.  The  consequence  was  that 
neither  of  us  saw  anything  of  the  total  eclipse.  Our 
labour  begun,  the  first  plate  was  exposed,  as  an  experi- 
ment, from  five  to  ten  seconds,  in  order  to  see  what  was 
the  proper  time  for  exposure. 

"  Mohammed,  our  dusky  attendant,  brought  the  first 
box  into  the  tent  to  me.  I  poured  the  iron  developer  upon 
the  plate,  waiting  breathlessly  to  see  the  result.  At  this 
moment  my  lamp  went  out.  '  Light !  Light ! '  I  ex- 
claimed ;  but  no  one  heard  me — every  one  had  enough  to 
do.  I  stretched  my  right  hand  out  of  the  tent,  holding 
the  plate  with  the  left,  and  fortunately  grasped  a  small 
oil  lamp,  which  I  had  placed  ready  for  all  emergencies, 
and  now  I  saw  the  image  of  the  sun  appear  upon  the 
plate.  The  dark  rim  of  the  sun  was  surrounded  by  a  series 


180  THE    CHEMISTRY    OF   LIGHT. 

of  peculiar  prominences  on  one  side,  while  on  the  other 
side  appeared  a  singular  horn, — both  phenomena  per- 
fectly analogous  in  both  views.  My  delight  was  great, 
but  there  was  no  time  for  rejoicing  ;  soon  the  second 
plate,  and,  a  minute  later,  the  third  plate  were  in  my 
tent.  'The  sun  is  emerging/  exclaimed  Zencker.  The 
total  eclipse  was  over ;  but  all  this  appeared  as  the  work 
of  a  moment,  so  quickly  had  the  time  passed..  The 
second  plate  showed  under  development  only  faint  traces 
of  the  view,  a  passing  veil  of  clouds  had  almost  destroyed 
the  photographic  operation  at  the  moment  of  exposure. 
The  third  plate  showed  again  two  successful  views  with 
protuberances  on  the  outer  rim. 

"  Eejoicing  in  this  success,  the  plates  were  washed, 
fixed,  varnished, — no  doubt  with  very  imperfect  materials, 
— some  copies  were  taken  on  glass,  and  these,  to  obviate 
loss,  were  transmitted  separately  to  Europe. 

"Our  extraordinary  good  fortune  is  apparent  from  the 
fact  that,  at  a  place  distant  only  half  a  league  from  our 
station,  nothing  was  seen  of  the  total  eclipse  on  account 
of  the  veil  of  clouds. 

"We  did  not  stay  long  at  Aden  after  our  chief  object 
had  been  attained :  in  three  days  the  steamer  proceeded 
to  Suez.  Telescope,  wheel  work,  and  our  heap  of  instru- 
ments and  chemicals  were  quickly  packed,  placed  on 
camels  and  conveyed  to  the  harbour.  On  the  21st  of 
August  we  bade  adieu  to  the  barren,  rocky  island,  and 
started  for  Suez." 

Aden  was  one  of  the  points  where  the  eclipse  was 
soonest  seen.  As  previously  stated,  the  English  had 
also  equipped  a  photographic  expedition,  which  stationed 
itself  at  Guntoor  in  India.  The  eclipse  was  observed  an 


THE   APPLICATIONS   OF   PHOTOGRAPHY.  181 

hour  later  in  India  than  at  Aden.  The  same  protube- 
rances appeared  in  the  Indian  photographs  as  in  those 
at  Aden,  but  they  present  a  very  different  form,  which 
seems  to  show  that  these  prominences  are  not  compact 
bodies,  but  formations  of  a  cloud-like  nature ;  and  this 
supposition  was  converted  into  certainty  by  Jansen's 
observations  with  the  spectrum,  made  simultaneously. 
Jansen  showed  that  in  a  total  eclipse  the  protuberances 
display  clear  lines  in  the  spectroscope  ;  but,  as  this  only 
takes  place  with  gaseous  bodies,  the  question  about  the 
nature  of  the  protuberances  was  solved.  Jansen  deter- 
mined at  the  same  time  the  exact  position  of  the  clear 
lines  of  the  spectrum,  and  detected  the  nature  of  the 
gaseous  substance  as  glowing  hot  hydrogen.  He  sub- 
sequently made  the  discovery  that  an  eclipse  was  by  no 
means  necessary  in  order  to  detect  the  clear  lines  of  the 
protuberances.  They  are  seen  on  clear  days,  if  the  eye- 
piece of  a  spectroscope  be  directed  to  the  sun's  rim,  and 
the  changeable  nature  of  these  protuberances  can  be 
observed  on  the  appearance  and  disappearance  of  these 
clear  lines.  Zollner  of  Leipzig  even  detected  this  sudden 
naming  up  through  the  spectroscope,  also  the  sudden 
breaking  away  of  gas  clouds  from  their  substratum, 
and  their  dispersion,  all  in  the  space  of  a  few  minutes. 

We  add  here  a  faithful  copy  of  the  Aden  photographs, 
which  we  have  taken  from  Herr  Schellen's  excellent 
work  on  spectral  analysis,  published  by  Westermann  at 
Brunswick.  The  first  view  gives  us  the  eastern  rim  of 
the  sun ;  the  western  was  covered  by  clouds.  It  is 
easy  to  recognize  in  it  the  large  horn-liktf  protuberance, 
which  has  an  elevation  of  184,000  miles,  and  gives  an 
idea  with  what  immense  force  masses  of  gas  are  pro- 


182 


THE    CHEMISTRY    OF    LIGHT. 


jected  over  the  surface  of  the  sun.  It  shows,  further, 
the  remarkable  protuberance  to  the  left,  in  which 
the  masses  of  gas  appear  like  powerful  jets  of  flame 
driven  sideways  by  a  tempestuous  wind ;  a  light  region 


surrounding  the  protuberances  forms  the  glowing  hot 
stratum  of  vapour  permanently  surrounding  the  rim, 
and  is  named  chromosphere. 

The  second  view  presents  only  a  series  of  point-like 
protuberances  on  the  western  rim  of  the  sun,  but  these 


THE    APPLICATIONS    OF    PHOTOGRAPHY. 


183 


points  are  so  large  that  our  earth  could  almost  find 
room  in  them.  The  eastern  part  of  the  sun  was  under 
the  clouds  during  the  taking  of  this  view. 

Finally,  the  third  view  gives  a  perfect  representation 
of  the  total  eclipse  as  it  was  observed  in  India.  Besides 
the  protuberances  seen  at  Aden,  there  is  another  on  the 


Fig.  75. 


western   rim  of  the  sun,  which  was  quite   covered   by 
clouds  at  Aden. 

Photography  has  been  latterly  applied  to  the  observa- 
tion of  total  eclipses  on  a  more  magnificent  scale.  Thus, 
on  the  7th  of  August,  1869,  hundreds  of  photographers 
were  actively  employed  in  observing  the  total  eclipse  of 


184 


THE    CHEMISTRY   OF   LIGHT. 


the  sun  at  Iowa,  in  North  America,  and  more  than 
thirty  telescopes  were  set  up  to  fix  the  phenomenon. 
By  these  observations,  the  question  respecting  the  nature 
of  the  protuberances  was  finally  set  at  rest,  and  the 
only  question  that  remained  related  to  the  corona.  By 

,  w 


Fig.  te. 

corona  is  implied  a  kind  of  nimbus  of  white  light 
encompassing  the  sun  when  totally  eclipsed.  Many 
observations  of  total  eclipses  have  been  undertaken  for 
the  solution  of  this  question.  A  very  beautiful  view  of 


THE   APPLICATIONS   OF   PHOTOGRAPHY. 


185 


the  corona  was  obtained  by  Whipple,  at  Shelbyville  in 
Kentucky,  August  7th,  1869.  A  much  longer  exposure 
is  required  in  the  case  of  the  corona  than  in  taking  the 
protuberances,  on  account  of  the  feeble  light  attending 
the  phenomenon.  At  Shelbyville,  the  exposure  for  the 
corona  lasted  forty-two  seconds,  whereas  five  seconds 


sufficed  to  take  the  protuberances.     Nor  was  the  nature 
of  the  corona  as  yet  determined. 

In  1870  the  English  sent  out  an  expedition  for  the 
observation  of  the  corona,  conducted  by  Lockyer,  to 
Catania,  and  the  author  accompanied  it.  Unfortunately, 
owing  to  the  unfavourable  weather,  the  observations  were 


183  THE    CHEMISTRY   OF   LIGFHT. 

only  partially  successful.  Nevertheless,  a  detachment  of 
the  expedition,  conducted  by  Brother,  to  Syracuse,  suc- 
ceeded in  obtaining  a  good  view  of  the  corona,  and  we 
give  a  faithful  woodcut  copied  from  this  photograph. 

The  black  prominences  round  the  sun's  disc  give  the 
situation  of  the  protuberances  which  wrere  visible  on  the 
day  of  the  eclipse.  But  we  lay  stress  on  the  fact  that 
they  are  not  visible  in  the  photograph  of  the  corona. 
To  take  a  view  of  the  corona  requires  an  exposure  eight 
times  as  long  as  the  protuberances.  During  this  long 
exposure  the  protuberances  in  the  view  received  too 
much  influence,  and  are  therefore  paler,  so  that  their 
outline  becomes  confounded  with  the  indistinct  parts. 

Photography  is  applied  to  other  important  objects  be- 
sides eclipses.  Views  of  the  sun  are  taken  daily  with  it. 
The  observation  of  centuries  has  established  that  the  sun 
is  continually  changing  :  spots  appear,  increase,  and  dis- 
appear. All  these  phenomena  were  at  an  earlier  date 
explained  as  openings  in  the  cloudy  luminous  atmo- 
sphere of  the  sun,  wrhich  was  supposed  to  surround  its 
dark  central  mass.  Now  they  are  looked  upon  as  im- 
mense whirlwinds,  which  rage  in  the  atmosphere  of  the 
sun  (see  Schellen's  "  Spectral  Analysis  "  page  200),  or  as 
cloud-like  condensations.  Their  nature  has  not  been 
perfectly  ascertained.  These  sun-spots  follow  the 
revolution  of  the  sun's  body  round  its  axis,  and  expe- 
rience manifold  changes  during  this  time.  It  has  been 
only  by  means  of  these  spots  that  the  duration  of  the 
sun's  revolution  has  been  determined.  Recent  observa- 
tions have  established  that  the  size  and  varying  number 
of  the  spots  change  periodically,  and  that  these  are 
connected  with  the  magnetic  phenomena  of  our  earth. 


THE    APPLICATIONS    OF    PHOTOGRAPHY. 


187 


These  circumstances  have  led  to  a  more  devoted  study 
of  the  spot  formations,  and  photography  has  offered  a 
valuable  aid  to  them.  It  gives  at  a  particular  moment 
a  faithful  view  of  the  sun's  surface,  and  photographs 
taken  daily  give  us  the  most  exact  representation  of  its 
spots,  their  size  and  number ;  and  a  comparison  of  the 
views  during  one  month  gives  an  instructive  survey  of 
the  changes  on  the  sun's  surface,  as  they  relate  more 
faithfully  than  words  the  history  of  the  central  body  of 
our  planetary  system.  The  amateur  Lewis  Eutherford, 
at  New  York,  who  has  made  valuable  contributions  to 


Fig.  78. 

astronomical  photography,  has  taken  a  great  number 
of  these  views  at  the  photographic  observatory  built  at 
his  own  expense. 

These  views,  taken  on  successive  days,  exhibit  manifold 
groups  of  spots,  often  of  considerable  size;  and  the 
change  in  their  form  and  position  is  accurately  dis- 
cerned (this  change  consequent  upon  the  revolution  of 
the  sun's  body).  These  impressions  are  not  prepared, 
as  were  the  pictures  of  the  eclipse,  in  the  principal  focus 
of  the  telescope,  but  in  an  appendage  (Fig.  78)  which 
answers  the  purpose  of  a  magnifying  apparatus.  This 
contains  a  small  lens  L,  which  projects  'on  the  ground 
glass  G  an  enlarged  image  of  the  small  representation 
of  the  sun  S  produced  by  the  great  lens  0. 


188  THE    CHEMISTRY   OF   LIGHT. 

In  this  manner  Eutherford  obtained  immediately  a 
view  of  the  sun  of  about  two  inches  diameter.  This 
enlarging  apparatus  is  not  to  be  recommended  in  the 
case  of  eclipses  of  the  sun,  for  the  clearness  of  the 
optical  image  produced  by  the  great  telescopic  lens  is 
materially  weakened  by  the  enlargement.  When  the 
enlargement  is  twice  as  great,  the  weakening  of  the  light 
is  fourfold;  when  it  is  three  times  as  great,  the  weakening 
is  ninefold,  and  so  on.  In  views  of  the  unclouded  sun 
this  is  of  no  consequence,  for  its  light  is  so  intense  that 
it  bears  a  considerable  enlargement,  and  yet  remains 
clear  enough  to  give  a  view  on  a  momentary  exposure. 
But  it  is  otherwise  with  the  protuberances,  which  give 
out  much  less  light,  and  which,  on  the  application  of  an 
enlarging  system,  would  produce  such  faint  views  that 
a  longer  exposure  would  be  required  than  the  duration, 
of  the  eclipse. 

The  solution  of  other  important  astronomical  pro- 
blems has  been  attempted  with  the  help  of  photography ; 
for  example,  the  production  of  views  of  the  starry 
heavens. 

The  object  of  these  views  of  the  stars  is  a  representa- 
tion of  the  constellations,  or  the  relative  position  of  the 
stars.  It  was  always  one  of  the  principal  objects  of 
astronomy  to  determine  the  position  of  the  fixed  stars. 
It  may  be  thought  that  the  catalogue  of  the  stars  is 
already  completed,  and  that  the  matter  is  settled;  but 
this  is  not  the  case.  As  far  as  photography  can  at 
present  be  applied,  that  is,  to  stars  of  the  ninth  magni- 
tude, the  catalogue  is  not  complete.  Moreover,  the 
measurements  of  the  past  may  require  correction  in 
consequence  of  improved  methods. 


THE   APPLICATIONS   OF   PHOTOGRAPHY.  189 

The  photographic  process  has  a  scientific  importance 
for  this  end,  because  it  offers  advantages  in  the  facility 
and  correctness  of  its  results.  Many  readers  may 
inquire  why  we  take  so  much  trouble  to  discover  with 
the  greatest  accuracy  the  positions  of  thousands  and 
millions  of  fixed  stars.  The  answer  is  that  the  fixed 
stars  are  not  stationary,  as  their  name  implies ;  nothing 
is  stationary  and  at  rest  in  nature,  and  hence  their 
study  is  never  at  an  end.  No  doubt  the  fixed  stars 
change  their  position  so  slowly  that  the  builders  of  the 
pyramids  four  thousand  years  ago  beheld  the  constel- 
lations much  as  we  do.  It  is  only  the  minutest 
astronomical  measurements  that  show  such  a  change 
within  a  limited  number  of  years.  However,  the  study 
of  the  proper  movement  of  the  fixed  stars  has  now 
begun,  and  requires  very  accurate  measurements  carried 
on  for  generations. 

Another  interesting  point  comes  into  consideration 
in  this  connection.  On  the  one  hand,  the  fixed  stars  are 
not  without  movement;  on  the  other,  their  distances 
from  the  earth  vary,  and  those  of  the  nearest  are 
immensely  great.  The  photographer  who  wishes  to 
have  a  graphic  view  of  an  object,  will  always  seek  to  take 
it  from  different  points.  Two  views  of  a  moderately 
remote  object,  taken  from  two  points  that  are  only  two 
inches  apart,  appear  different  to  the  eye,  and  produce, 
when  viewed  in  a  special  manner,  a  stereoscopic  effect. 
No  distance  on  earth  is  great  enough  to  give  different 
pictures  of  the  same  constellation ;  nevertheless,  within 
the  space  of  one  half-year  we  describe  a, circle  round  the 
sun  having  a  diameter  of  184  millions  of  miles,  so 
that  in  half  a  year  we  are  184  millions  of  miles  from  our 


190  THE    CHEMISTKY   OF   LIGHT. 

present  position.  This  enormous  distance  is  in  certain 
cases  sufficient  to  show  a  change  in  the  mutual  position 
of  certain  stars,  though  the  distance  is  insufficient  for 
the  naked  eye  and  the  stereoscope,  and  only  available 
for  the  finest  astronomical  instruments.  By  this  means 
the  distance  of  the  nearest  fixed  stars  has  been  deter- 
mined, amounting  to  billions  of  miles. 

By  careful  comparative  measurements  of  positions  of 
neighbouring  stars,  continued  for  years  and  centuries,  a 
change  can  be  proved  to  exist,  and  the  proper  movement 
of  the  stars  can  be  calculated.  The  distance  of  the 
stars  can  be  deduced  by  a  careful  collating  of  the 
yearly  recurring  changes  in  the  positions  of  the  stars. 
It  is  apparent  that  the  fixing  of  these  positions  by 
photography,  which  admits  the  taking  of  measurements 
at  any  given  time,  must  be  of  the  greatest  value  for 
both  these  astronomical  problems. 

The  photographing  of  the  stars  was  first  introduced 
into  science,  about  twenty  years  ago,  by  Professor  Bond, 
of  Cambridge,  Massachusetts,  but  it  was  Mr.  Lewis 
Eutherford,  of  New  York,  who  perfected  this  method. 
He  constructed  a  photographic  objective  of  11  inches 
diameter  and  about  13  feet  focus.  This  objective  shows 
a  considerable  focal  difference ;  that  is,  the  violet  and 
blue  rays  have  a  different  focus  from  the  yellow  and 
red.  If  a  clear  image  of  the  star  is  taken,  the  sensitive 
plate  is  adjusted  to  the  focus  of  the  yellow  rays,  and 
the  chemically  operative  blue  rays  are  then  situated 
outside  the  sensitive  plate,  and  produce  a  faint  impres- 
sion. The  plate  must  therefore  be  adjusted  to  the 
focus  of  the  blue  rays ;  but  this  is  not  so  easy  to  find. 
After  it  has  been  found  approximatively,  it  is  corrected 


THE   APPLICATIONS   OF    PHOTOGRAPHY.  191 

by  taking  different  photographs  of  the  star,  changing 
the  position  of  the  plate.  The  point  is  determined  from 
which  the  best  and  sharpest  view  is  taken,  and,  by 
continual  repetitions  of  the  attempts,  the  chemically 
operative  focus  of  the  lens  of  13  feet  focal  distance  can 
be  accurately  determined  to  within  TJo  of  an  inch. 
It  is  well  known  that  all  heavenly  bodies  have  the  same 
focus,  on  account  of  their  great  distance.  No  photo- 
graphic objective  gives  a  picture  with  a  large  surface 
perfectly  correct.  Accordingly,  with  the  accuracy 
required  by  astronomic  photography,  the  surface  to  be 
devoted  to  the  image  can  only  be  very  small,  or  about 
1 J  degrees.  Errors  in  drawing,  which  must  appear  here, 
are  controlled  and  corrected  by  photographing  a  very 
correct  scale,  and  comparing  the  picture  with  the  original. 
A  field  of  1 J  degrees,  or  three  times  the  moon's  diameter, 
embraces  the  well-known  image  of  the  Pleiades. 

The  telescope  of  Eutherford  is  arranged  as  in  Fig.  73 
(p.  173) ;  it  is  moved  by  clockwork,  to  be  able  exactly  to 
follow  the  movement  of  the  stars. 

The  views  of  large  stars  taken  with  it,  after  a  short 
exposure,  all  appear  like  small  round  points  that  can 
only  be  seen  through  the  magnifying  glass.  In  the  case 
of  a  long  exposure  their  size  depends,  fundamentally, 
on  the  more  or  less  strong  vibrations  of  the  atmo- 
sphere, which  occasion  the  nickering  of  the  stars. 
Stars  of  the  ninth  magnitude  can  be  photographed  with 
an  exposure  of  eight  minutes ;  these  stars  are  ten  times 
weaker  than  the  faintest  that  can  be  detected  on  a 
clear  night  by  the  naked  eye,  and  their  images  are  very 
small  points.  It  would  be  difficult  to  distinguish  these 
small  points  from  dirt  spots  on  the  plate.  To  do  this, 


192  THE   CHEMISTRY   OF   LIGHT. 

Rutherford  makes  use  of  an  ingenious  process.  He 
brings  the  telescope,  after  the  first  exposure  of  eight 
minutes,  into  a  slightly  different  direction,  and  makes 
another  exposure  of  eight  minutes,  while  the  clock- 
work continues  to  operate,  and  moves  the  telescope 
correctly  in  this  second  direction. 

In  this  manner  two  images  are  obtained  of  every  star 
on  the  plate,  closely  adjacent ;  the  distance  and  relative 
position  being  in  all  the  same.  These  double  views 
can  be  easily  found  on  the  plate  and  distinguished  from 
spots.  If  the  telescope  stops,  it  is  evident  that  the 
images  of  the  stars  make  a  movement  on  the  plate,  the 
bright  stars  describing  a  line.  This  line  is  of  great 
importance  to  determine  the  direction  from  east  to 
west  on  the  plate.  For  faint  stars  which  leave  no  line  a 
third  exposure  is  necessary,  to  determine  this  direction ; 
the  same  thing  takes  place  after  the  clockwork  of  the 
telescope  has  been  stopped  for  some  minutes. 

Eutherford  has  already  taken  numerous  views  of  the 
stars,  and  they  will  serve  as  important  means  of  com- 
parison, after  the  lapse  of  centuries,  in  order  to  discover 
what  change  has  taken  place  in  the  position  of  the  fixed 
stars.* 

But  another  heavenly  body  invites  us  specially  to 
study  it  by  the  help  of  photography  ;  that  is  our 
neighbouring  satellite,  the  moon.  The  unassisted  eye 
recognizes  its  uneven  surface  ("  mountains  in  the 
moon")  and  the  varying  shades  of  its  ground  (moon 
spots).  Its  surface  contains  a  thousand  problems, 
appearing  as  a  rigid,  almost  vitreous,  waterless,  airless 
waste. 

*  Details  respecting  Rutherford's  observatory  are  contained  in  the 
"  Photographischen  Mittheilungen,"  Jahrg.  1870.  Berlin  :  Oppenheim. 


THE   APPLICATIONS   OF   PHOTOGRAPHY.  193 

Warren  de  la  Eue  tried  to  take  this  singular  globe, 
which  is  so  near  to  our  earth  and  yet  so  different;  he 
actually  prepared  a  small  view  of  the  moon  with  the 
help  of  a  telescope,  which  he  enlarged  to  24  inches 
with  the  aid  of  an  enlarging  apparatus  (p.  96). 

The  moon  gives  out  less  light  than  the  sun.  It  is 
therefore  taken  to  the  best  advantage  in  the  principal 
focus  of  the  telescope.  (See  Fig.  72,  p.  172.)  In  the 
most  favourable  case,  three-quarters  of  a  second  suffices 
for  exposure,  but  it  is  rare  to  obtain  sharp  negatives, 
owing  to  the  disturbance  of  the  atmosphere.  Therefore, 
to  obtain  a  sharp  image  of  the  moon  is  a  test  of  patience. 
After  Warren  de  la  Eue,  Eutherford  obtained  notoriety 
by  taking  moon-pictures ;  his  improved  telescope,  set  up 
purposely  for  photographic  purposes,  gave  a  still  sharper 
image  of  the  moon  than  De  la  Eue's,  and  our  frontispiece 
is  a  diminished  copy  of  the  enlarged  picture  of  the  moon 
according  to  the  original,  for  which  we  are  indebted  to 
Eutherford,  forming  a  genuine  map  of  the  moon  of  no 
small  importance  to  astronomy. 

Some  years  ago  Schmidt,  at  Athens,  maintained  that 
a  volcano  given  out  by  Madler  as  extinguished  is  no 
longer  to  be  found,  and  he  thereby  proved  the  possibility 
of  changes  on  the  apparently  rigid  surface  of  the  moon. 
If  a  photograph  of  the  moon's  surface  could  have  been 
taken  forty  years  ago,  when  Madler  observed  the  volcano, 
we  should  now  be  certain  about  this  point,  which  is  still 
hypothetical. 

But  the  sun  and  its  eclipses,  the  moon,  and  stars  are 
not  the  only  objects  of  astronomic  photography.  Its 
province  extends  further  since  the  discovery  of  spectral 
analysis. 


194  THE    CHEMISTEY   OF    LIGHT. 

"When  it  was  discovered  that  those  wonderful  lines 
intersecting  the  sun's  spectrum  (see  chap.  vii.  p.  63) 
were  occasioned  by  glowing  substances  of  different 
nature,  and  that  each  element  shows  invariably  the 
same  lines,  so  that  the  presence  of  certain  spectral  lines 
establishes  the  presence  of  certain  substances,  it  became 
necessary  to  possess  an  exact  view  of  the  solar  spectrum, 
with  all  its  countless  lines.  This  was  essential,  in  order 
to  be  able  at  once  to  see  what  substances  are  yielded 
by  these  lines,  by  comparing  this  view  with  the  spectrum 
of  a  flame,  or  of  a  fixed  star.  Kirchhoff,  one  of  the 
discoverers  of  spectral  analysis,  and  Angstrom  have 
prepared  such  a  detailed  view  of  the  solar  spectrum. 
Their  labour  would  have  been  materially  simplified 
if  Rutherford  had  published  his  photograph  of  the 
spectrum  a  year  earlier. 

I  admit  that  this  photographic  spectrum  of  Ruther- 
ford's only  shows  the  lines  of  the  photographically 
operative  part  of  the  spectrum — from  green  to  violet 
— but  it  does  this  with  wonderful  clearness.  Many  lines 
that  appear  faint  to  the  naked  eye  show  themselves 
strong  and  sharp  in  the  view ;  nay,  lines  are  discovered 
in  the  photograph  of  the  spectrum  which  Kirchhoff  did 
not  see  in  the  solar  spectrum. 

The  causes  of  this  phenomenon  may  be  twofold :  either 
the  eye  does  not  take  in  certain  rays  of  light  by  certain 
lines, — as  we  know  it  is  not  influenced  by  the  ultra-violet 
rays,  which  have  a  strong  photographic  effect, — or  it  is 
possible  that  changes  take  place  in  the  sun,  that  at 
certain  times  fresh  substances  come  to  its  surface,  and 
thereby  new  lines  become  apparent. 

The  taking  of  a  spectrum  is  effected  with  the  help  of 


THE   APPLICATIONS    OF   PHOTOGRAPHY. 


195 


an  ordinary  spectral  apparatus,  seen  in  Fig.  79.  This 
consists  of  a  tube  A,  which  has  a  fine  slit  F,  through 
which  the  light  penetrates.  At  the  end  of  the  pipe  is  a 
lens,  which  makes  all  the  rays  issuing  from  the  slit 
parallel,  and  conducts  them  to  the  prism  P.  This 
refracts  the  rays  in  such  wise  that  they  fall  into  the 
tube  J3,  and  can  be  observed  through  its  narrower  end. 


Fig.  79. 

If  the  object  is  to  photograph  the  spectrum  thus  seen,  an 
opaque  photographic  camera  is  placed  close  to  the  tube, 
its  eye-piece  is  drawn  a  little  out,  and  then  the  image  of 
the  spectrum  appears  upon  the  ground-glass  slide. 

Attempts  have  been  made  to  solve  other  important 
problems  by  the  help  of  photography.  Thus,  Dr. 
Zencker  hoped  to  be  able  to  trace  the  path  of  falling 
stars  by  means  of  it.  Unfortunately,  these  were  found  to 


196 


THE    CHEMISTBY   OF   LIGHT. 


give  out  too  little  light  to  produce,  while  they  lasted,  an 
impression  on  the  photographic  plate. 

A  grand  new  era  is  before  photography  in  observing 
the  transit  of  Venus. 

In  determining  the  distance  of  heavenly  bodies,  the 
earth's  orbit  is  taken  as  base ;  therefore  the  knowledge 
of  the  exact  amount  of  this  base  is  assumed.  Now  this 
amount  has  only  hitherto  been  determined  by  approxi- 
mation, and  is  in  round  numbers  one  hundred  and  sixty 
millions  of  miles. 

It  has  long  been  an  effort  to  determine  this  number 
more  accurately,  but  to  do  so  is  attended  with  great 
difficulty.  Let  it  be  conceived  that  there  are  at  two 
different  points  of  the  earth,  a  and  b  (Fig  80),  two 
observers  who  look  with  telescopes  at  a 
star  x,  and  measure  the  angle  which  the 
eye-line  makes  with  the  line  a  b ;  it  can  be 
determined  from  both  angles  and  the  line 
a  b  (which  is  easily  found)  what  the  dis- 
tance of  the  star  is  from  a  or  b.  This  is 
the  trigonometrical  method,  and  it  gives 
certain  reliable  results,  if  the  distance  of 
the  star  is  not  too  great ;  thus,  for  example, 
,  the  distance  of  the  moon,  which  is  about 
-ten  of  the  earth's  diameter,  is  easily 
ascertained.  If  the  star  to  be  measured 
is  too  remote,  the  eye-lines  a  x  and  b  x 
are  nearly  parallel,  no  difference  exists 
between  the  two  angles  at  a  and  b,  and 
the  trigonometrical  method  is  useless.  This  is  the  case 
with  the  sun,  which  is  ninety-five  millions  of  miles 
from  the  earth.  We  can  therefore  only  ascertain  its 
distance  by  indirect  methods. 


Fig.  80. 


THE   APPLICATIONS   OF   PHOTOGRAPHY.  197 

According  to  a  law  discovered  by  the  celebrated 
astronomer  Kepler,  the  squares  of  the  periods  of  revolu- 
tion of  the  planets  are  in  proportion  to  the  cubes  of  their 
distances  from  the  sun.  Thus,  if  the  period  of  the 
earth's  revolution  is  U,  that  of  Venus  u,  the  distance  of 
the  earth  E,  that  of  Venus  e,  according  to  this  law 

U2  :  u*  =  E3:  e3. 
If  the  cube  root  is  extracted  from  both  we  have — 

3  3    

Vu*  =  E  :  e,  hence, 

3  _       3 

v^a  :    Vu*  =  E  -  e  :  c. 

But  E  —  e  is  the  distance  between  the  earth  and  Venus. 
When  this  has  been  determined  by  measurement,  three 
terms  of  the  proportion  are  known,  for  the  periods  of  the 
revolutions  of  Venus  and  the  earth  are  accurately  known. 
Then,  by  the  simple  rule  of  three,  the  fourth  term  e 
can  be  found ;  that  is,  the  distance  of  Venus  from  the 
sun.  If  to  this  is  added  the  distance  of  the  earth  from 
Venus,  we  obtain  the  distance  of  the  earth  from  the  sun, 
which  was  required. 

Thus  the  determination  of  the  distance  from  the  sun 
depends  on  that  of  the  distance  of  Venus,  which  must  be 
taken  at  the  moment  when  Venus  is  between  the  earth 
and  the  sun.  But  Venus  is  not  visible  at  the  moment 
when  it  is  placed  before  the  sun's  disc.  This  is  only 
exceptionally  the  case — twice  in  every  century — and 
then  it  appears  as  a  small  black  point  on  the  sun's  disc, 
which,  however,  continually  changes  place,  on  account  of 
the  earth's  movement  and  its  own.  This  circumstance 
renders  difficult  the  taking  simultaneous  measurements 
at  two  different  and  remote  points  of  the  earth,  and 


198 


THE    CHEMISTRY   OF    LIGHT. 


therefore  the  idea  has  been  entertained  of  using  photo- 
graphy as  an  auxiliary.  If  by  its  help,  and  in  the 
manner  described  above,  a  sun  picture  is  taken  during 
the  transit  of  Venus,  the  distance  of  Venus  from  the 
sun's  centre  can  be  easily  measured  upon  it.  The  centre 
of  the  sun  is  a  fixed  point  that  can  be  assumed  to  be 
stationary. 

If  the  earth  is  supposed  to  be  E  (Fig.  81),  Venus  at  F, 
and  the  sun  at  S,  the  observer  at  a  will  see  Venus  under 
the  centre  of  the  sun,  underlying  it,  while  an  observer  at 


Fig.  81. 

I  will  see  Venus  above  it.  Accordingly,  Venus  will 
present  a  different  position  to  the  sun's  centre  on  photo- 
graphs at  various  points  of  the  earth. 

Now,  the  situation  of  the  line  of  inclination  of  the 
sun's  centre  is  accurately  known.  The  diameter  of  the 
sun  corresponds  to  an  angle  of  thirty  minutes.  If  the 
sun's  diameter  is  supposed  to  be  divided  into  sixty  parts, 
each  part  corresponds  to  the  arc  of  a  minute ;  therefore, 
it  is  only  necessary  to  measure  the  number  of  such 
parts,  separating  Venus  from  the  sun's  centre,  and  we 
find  directly  the  angle  which  the  direction  of  the  eye- 
line  of  Venus  (for  example,  a  b)  makes  with  the  direction 
of  the  eye-line  of  the  sun's  centre  a  m.  If  this  angle 
is  drawn  from  the  angle  which  the  eye-line  of  the  sun's 
centre  makes  with  a  b,  we  obtain  the  angle  which  the 
eye-line  of  Venus  makes  with  the  line  a  b,  and  that 


THE   APPLICATIONS   OF   PHOTOGRAPHY.  199 

gives  all  the  data  necessary  to  calculate  the  distance  of 
Venus,  and,  from  that,  the  distance  of  the  central  body 
which  forms  the  foundation  and  base  of  all  astronomi- 
cal measurements.* 

The  determining  of  the  angle  by  photography  is  of 
special  value,  as  this  measurement  can  easily  be  made, 
and  at  any  convenient  time,  whereas  measurements  of 
the  star  itself  can  only  be  made  while  the  phenomenon 
is  visible,  and  hence  many  errors  are  introduced  in  the 
agitation  of  the  moment.  It  is  natural  that  measure- 
ments of  this  kind  require  apparatus  of  the  most 
accurate  description,  and  the  adoption  of  many  pre- 
cautions; therefore  preliminary  experiments  have  now 
been  begun  to  determine  the  degree  of  accuracy  which 
a  measurement  by  means  of  photography  admits.  If 
these  preliminary  experiments  give  a  favourable  result, 
numerous  photographic  expeditions  will  be  sent  out  to 
observe  the  transit  of  Venus.  Germany  proposes  to 
occupy  five  stations :  Tschifu  in  China,  Muscat  on  the 
Persian  Gulf,  Kerguelen's  Land,  and  the  Auckland 
Islands.  Besides  these,  England,  France,  Eussia,  and 
America  are  equipping  photographic  expeditions  which 
will  occupy  different  points,  and  thus  we  may  hope, 
though  some  stations  are  visited  by  unfavourable 
weather,  still  to  obtain  numerous  plates  by  means  of 
which  the  great  astronomical  question  can  be  solved. 

*  Oar  space  does  not  permit  ns  to  enter  into  the  details  of  the  method 
of  determining  the  sun's  parallax  ;  it  is  only  our  purpose  to  give  a  plain 
intelligible  statement  of  the  principle  of  the  thing.  Those  who  are 
specially  interested  in  the  subject  are  referred  to  Dr.  Schorr's  "  The 
Transit  of  Venus  over  the  Sun."  Brunswick :  Vieweg.  1873. 


200 


THE    CHEMISTRY   OF   LIGHT. 


SECTION  V. 
THE  PHOTOGRAPHIC  OBSERVATION  or  SCIENTIFIC  INSTRUMENTS. 

Observations  with  the  Thermometer  and  the  Barometer — Neumeyer's 
Apparatus  to  determine  the  Depth  of  the  Sea. 

Meteorological  observations  require  a  daily  repeated 
reading  of  the  barometer  and  the  thermometer.  To 
economize  this  reading,  and  yet  to  receive  a  perfectly 
safe  register  of  the  state  of  the  thermometer  and 
barometer  at  each  minute,  photography  has  been  turned 
to  account.  Let  the  reader  imagine  behind  the  tube  of 
a  thermometer  R  (Fig.  82)  or  barometer  a  drum,  which 
revolves  round  its  axis  a  by  means  of  clockwork.  Let 
sensitive  paper  be  wrapped  round  this 
drum,  and  the  whole,  except  the  ther- 
mometer, be  enclosed  in  a  cylinder  S 
which  has  only  a  small  slit  behind  the 
thermometer,  through  which  the  light 
can  penetrate.  The  upper  part  of  the 
thermometer  will  let  the  light  through, 
while  the  thread  of  quicksilver  will  stop 
the  light.  Therefore  the  strip  of  paper 
Fig.  82.  above  the  quicksilver  will  blacken,  and 
the  limit  of  the  blackening  on  the  paper  will  rise  and 
fall  with  the  mercury.  Now  the  time  can  be  marked 
beforehand  on  the  paper.  As  the  drum  revolves  once 
in  twenty-four  hours,  the  strip  of  paper  need  only  be 
divided  perpendicularly  into  twenty-four  parts,  and  the 
first  part  be  moved  opposite  the  thermometer  directly 
the  clock  strikes  twelve,  after  which  the  whole  may 
be  allowed  to  revolve.  Then  the  coloured  strip  shows 


THE   APPLICATIONS   OF   PHOTOGRAPHY.  201 

the  height  of  the  thermometer  at  all  times  of  the  day. 
In  the  same  manner,  the  height  of  the  barometer  can 
be  registered  by  photography. 

Professor  Neumeyer  has  latterly  employed  a  similar 
instrument  to  determine  the  temperature  in  the  depths 
of  the  sea.  As  there  is  no  light  producing  chemical 
effects  at  those  depths,  Dr.  Neumeyer  sends  down  a 
light-producing  apparatus.  This  consists  of  a  galvanic 
battery,  and  a  Giesler's  tube ;  that  is,  a  pipe  in  which 
very  attenuated  nitrogen  gas  is  enclosed,  and  through 
which  the  electric  current  is  passed.  Then  the  tube  gives 
out  a  faint  light.  But  this  faint  light  works  chemically 
very  powerfully,  because  it  contains  many  of  the  invisible 
ultra-violet  rays  (see  p.  64),  and  in  three  minutes  it  effects 
the  blackening  of  the  paper.  Neumeyer  also  attempts 
to  determine  with  his  apparatus  the  direction  of  the 
oceanic  currents.  For  this  purpose  the  apparatus  has 
an  appendage  not  unlike  a  vane,  and,  as  when  suspended 
to  a  cable,  it  can  be  conveniently  turned  in  all  direc- 
tions. If  oceanic  currents  occur,  it  can  be  so  placed  in 
the  depths  that  the  vane  is  parallel  to  the  current.  A 
magnetic  needle,  within  a  tube  impervious  to  water,  is 
enclosed  in  the  apparatus,  and  moves  over  a  disc  of 
sensitive  paper ;  this  magnetic  needle  points,  of  course,  to 
the  north,  and  the  luminous  tube  above  it  marks 
exactly  its  position  on  the  sensitive  paper,  which  is 
firmly  fastened  to  the  box.  Therefore,  it  can  be  easily 
seen  what  situation  the  apparatus  has  assumed  with 
reference  to  the  magnetic  needle — that  is,  Jthe  north. 


202  THE    CHEMISTRY   OF   LIGHT. 

SECTION  VI. — PHOTOGRAPHY  WITH  REFERENCE  TO  MEDICAL  RESEARCH. 

Photographs  of  the  Interior  of  the  Ej^e,  the  Ear,  etc. — Stem's 
Heliopictor, 

Photography  has  been  begun  to  be  applied  on  a  large 
scale  to  the  province  of  medical  science,  not  only  in 
taking  interesting  anatomical  preparations  and  morbid 
phenomena  of  short  duration,  but  in  giving  exact 
anatomical  views  of  the  different  organs.  The  ap- 
parently impenetrable  interior  of  living  organs  has 
been  disclosed  by  eye-mirrors,  ear-mirrors,  and  throat- 
mirrors,  so  that  their  interior  is  fully  disclosed  to  the 
eye  of  the  observer.  In  like  manner,  the  image  visible 
to  the  eye  has  been  successfully  retained  by  photo- 
graphy. Dr.  Stein,  of  Frankfort-on-the-Maine,  has 
done  good  service  in  this  branch,  not  only  as  a  practical 
photographer,  but  also  by  the  construction  of  suitable 
apparatus.  It  would  exceed  the  limits  of  this  book  to 
describe  all  the  apparatus  necessary  for  this  purpose. 
We  shall  content  ourselves  with  the  description  of  one, 
that  for  taking  the  interior  of  the  ear. 

The  apparatus  consists  of  three  parts  :  1st,  the  ear- 
funnel  A  ;  2nd,  the  lighting  apparatus  B ;  3rd,  the 
photographic  apparatus  D,  with  the  lenses  C  (Fig.  83). 
These  parts  are  placed  together,  as  may  be  seen  by  the 
accompanying  diagram.  The  apparatus  is  fastened  by  a 
joint  to  a  corresponding  stand,  in  order  to  give  it  the 
proper  direction,  according  to  the  position  of  the  sun. 
The  ear-funnel  A  is  a  conical  tube  about  1J  inches  in 
length,  to  push  aside  the  small  hairs  which  interrupt 
the  view  ;  it  is  made  of  vulcanized  india-rubber.  The 
lighting  apparatus  B,  which  is  easily  closed  by  a  cover 


THE    APPLICATIONS    OF    PHOTOGRAPHY. 


203 


at  a  d,  consists  of  two  metal  pipes,  soldered  together 
at  a  right  angle  at  b  c,  of  which  one  is  provided  with 
parallel  sides,  and  the  other  with  curved  sides.  At  the 
place  where  the  two  tubes  unite  is  a  perforated  aplanatic 
metal  mirror  (e  g  /),  inclined  at  an  angle  of  45°. 

The  photographic  apparatus  c  consists  of  a  double 
objective  C  of  twelve  lines,  besides  a  small  camera,  two 
inches  deep.  The  ground-glass  shade  X,  and  the  box 
Y,  are  adjusted  in  a  rectangle  D,  easily  moved.  An 
enlarging  plano-convex  lens  is  situated  between  the 
objective  and  the  lighting  apparatus  at  h.  According  to 
the  position  of  the  sun,  of  a  bright  cloud,  or  any  other 


Fig.  83. 


point  of  light,  the  lighting  apparatus  B  can  be  moved 
by  turning  round  on  its  axis;  so  that,  in  conjunction 
with  the  joint  of  the  stand,  the  apparatus  can  be  turned 
easily  and  firmly  in  all  directions. 

The  rays  which  penetrate  into  the  tube  B  are  thrown 
by  the  perforated  plane  mirror  e  f  in  the  direction  of  A 
on  the  drum  of  the  ear.  Eeflected  thence,  they  pass  at 
g  the  perforated  plane  mirror,  and  the  image  of  the  drum 
of  the  ear  is  thrown  on  the  ground-glass  slide  n  o,  by 
the  combination  of  the  lenses  of  the  objective  hikl  in. 


204 


THE    CHEMISTRY   OF   LIGHT. 


The  firm  fixing  takes  place  partly  by  means  of  the  screw 
of  the  objective  at  p,  partly  by  moving  the  lens  at  A, 
according  as  an  enlarged  image  or  one  of  life-size  is 
desired.  During  the  photographic  process,  an  assist- 
ant must  pull  the  ear  muscle  backwards  and  upwards, 
in  order  to  give  a  proper  direction  to  the  funnel  in 
the  tortuous  aperture  of  the  ear.  The  time  of  exposure 
in  the  sunlight,  if  a  good  collodion  of  iodide  of  bromium 
is  employed,  lasts  half  a  second;  under  bright  clouds  on 
a  clear  day,  from  five  to  ten  seconds,  according  to  the 
intensity  of  the  light.  The  opening  and  shutting  of  the 
apparatus,  to  favour  the  operation  of  the  rays  of  light, 
is  effected  at  a  d. 

Dr.  Stein  has  constructed,  as  an  aid  to  naturalists  and 
physicians,  a  very  pretty  instrument,  called  the  helio- 
pictor,  which  admits  of  taking  views  on  moist  plates 
without  the  dark  chamber.  The  heliopictor  is  a  kind 
of  box  which  can  be  placed  at  the  back  of  every  camera. 
Dubroni,  of  Paris,  first  constructed  such  developing 
boxes.  This  box,  a  section  of  which  is  given  in  the 
diagram  below,  contains  a  glass  receiver  K,  in  which  a 
silver  solution  can  be  poured  through 
a  stopcock,  not  visible  in  the  figure. 
The  glass  plate  to  be  prepared  is  covered 
with  collodion,  then  brought  through  the 
door  into  the  box  placed  at  the  aperture 
O  of  the  glass  case,  and  the  door  is 
shut.  Then  the  spring  a  presses  the 
water-tight  plate  p  against  the  glass 
receiver.  After  this,  the  box  is  turned 
over  to  the  right,  the  silver  solution 
Fig.  84.  flows  over  the  plate,  and  renders  it 


THE   APPLICATIONS    OF    PHOTOGRAPHY.  205 

sensitive.  The  continuation  of  the  operation  is  observed 
through  a  yellow  glass  slide  S,  which  admits  no  chemical 
light.  After  the  plate  has  been  properly  sensitized,  the 
box  is  again  placed  upright,  and  brought  into  the  camera 
instead  of  the  ground-glass  slide,  S  is  drawn  up,  and 
light  is  admitted.  Then  the  silver  solution  is  drawn 
off  through  a  stopcock,  and  a  solution  of  green  vitriol 
turned  on  instead ;  by  tilting  the  box  this  flows  over  the 
plate  and  develops  the  picture.  Its  coming  out  is 
observed  through  the  yellow  slide  S.  After  the  develop- 
ing, the  picture  is  taken  out  and  fixed. 

Stein  improved  the  developing  box,  by  substituting  a 
vulcanized  india-rubber  receiver,  easily  taken  out  and 
easy  to  clean,  in  the  place  of  the  glass  receiver.  He 
also  introduced  the  method  of  filling  and  emptying  the 
receiver  by  means  of  a  stopcock,  Dubroni  having 
employed  pipettes.  Both  apparatus  are  in  the  "  Photo- 
graphischen  Mittheilungen,"  JahrgangX.  Nr.  117,  118. 

SECTION  VII. — PHOTOGRAPHY  AND  THE  MICROSCOPE. 
On  Microscopes — Taking  Microscopic  Views  —Their  Application. 

Nowhere  has  photography  shown  itself  a  more 
brilliant  auxiliary  or  substitute  for  the  art  of  drawing 
than  in  the  reproduction  of  miscroscopic  objects.  This 
field  was  worked  at  the  earliest  period  of  art,  for 
Wedgwood  and  Davy  strove  to  retain  the  images  of  the 
solar  microscope  by  the  help  of  sensitive*  silver  paper. 
This  solar  microscope  seemed,  in  fact,  to  be  made  for 
photographic  purposes.  It  consists  principally  of  a 
microscopic  object,  which  is  inserted  at  in,  and  is  either  a 
drop  of  liquid  brought  upon  a  glass  plate,  or  a  small  solid 
body  compressed  between  two  thin  glass  plates  (Fig.  85). 


206 


THE    CHEMISTRY   OF   LIGHT. 


The  small  lens  projects  an  enlarged  image  of  this 
minute  object  accurately  on  an  opposite  screen,  or  a 
white  wall,  exactly  as  shown  in  the  following  figure. 

The  screw  at  D  serves  to  approach  or  remove  the 
lens  from  the  object  m,  and  thereby  to  throw  out  the 
object  shafply  upon  the  screen.  E  is  a  dark  window, 
by  which  the  rim  of  the  round  image  is  cut  off. 


Fig.  85 

The  principal  part  of  the  image  B  C  contains  the  lens 
that  transmits  the  light.  Each  considerable  enlarge- 
ment diminishes  the  light  of  the  picture  materially ; 
if  it  is  enlarged  three  times,  the  clearness  is  diminished 
to  J ;  if  enlarged  fourfold,  to  ^ ;  if  fivefold,  to  ^ ;  if  a 
hundredfold,  to  joion.  With  such  a  diminution  of  bright- 
ness the  eye  would  not  detect  anything,  if  care  were 
not  given  to  throw  an  intense  light  on  the  object.  The 
system  of  lenses  contained  in  the  pipe  B  C  answers  this 
purpose.  This  concentrates  the  sun's  rays,  which  are 
reflected  by  mirror  M  into  the  tube  B,  on  the  micro- 
scopic object;  and  the  latter  becomes  in  this  manner  so 


THE   APPLICATIONS    OF   PHOTOGRAPHY. 


207 


well  lighted  that  it  admits  of  any  amount  of  enlarge- 
ment. The  room  in  which  the  instrument  is  placed  is 
dark;  accordingly,  all  the  conditions  are  present  that 
enable  proper  photographs  to  be  taken.  All  that  is  requi- 
site is  to  place  a  sensitive  plate  instead  of  the  image. 


Fig.  86. 

Few  persons,  however,  possess  a  solar  microscope. 
For  ordinary  investigations  a  microscope  is  used  similar 
to  that  in  the  accompanying  figure.  It  contains  at  o  a 
system  of  enlarging  lenses,  which  projects  an  enlarged 
image  S  R  of  the  small  object  r  s, 
as  shown  in  Fig.  88.  This  is  viewed 
through  the  eye-piece  c  d  (Fig.  88),  which 
is  placed  at  n  (Fig.  87),  and  enlarges  for 
the  second  time  the  image  S  R,  so  that 
a  still  greater  image  S '  R '  (Fig.  88)  is 
produced. 

This  is  seen  directly  by  the  eye  of  the 
observer.  The  necessary  light  is  thrown 
on  the  object  by  the  help  of  a  concave 
mirror  s  s  (Fig.  87). 

To  produce  photographs  with  the  help 
of  such  a  microscope,   a   photographic 
camera   can   be    placed  in    a    straight 
line   with    the    eye-piece    n  (Fig.   87), 
Fig.  87.  by    supporting    it    on    a    three-legged 


208 


THE    CHEMISTRY   OF   LIGHT. 


stand.  This  camera  does  not  require  a  lens,  like  camera 
p.  90,  bat  only  a  slit  through  which  the  opaque  tube  n 
passes.  The  eye-piece  n  (Fig.  87)  is  fixed  into  a  sleeve- 
like  appendage  which  surrounds  the 
slit,  and  then  the  tube  h  is  raised 
slightly. 

By  this  means  the  enlarged  image 
of  the  object  situated  at  S  R  (Fig.  88) 
is  visible  on  the  ground-glass  slide 
of  the  camera,  and  can  be  easily 
photographed. 

It  is  necessary  in  doing  this  that 
all  light  not  emitted  from  the  object 
should  be  excluded.  If  the  mirror 
s  sf  (Fig.  87)  throws  light  on  the 
object,  many  rays  pass  beside  it,  fall 
on  the  lenses,  and  occasion  reflec- 
tions that  materially  disturb  the  purity  of  the  image.  In 
this  case  it  is  advantageous  to  insert  a  system  of  lenses 
between  mirror  s  s  and  the  object,  concentrating  all  the 
rays  on  the  object. 

Instead  of  sunlight,  recourse  is  had  to  artificial  light ; 
for  example,  electric  and  magnesium  light,  which  makes 
the  observer  independent  of  the  weather.  The  beauty 
of  the  micro-photograph  depends  essentially  on  the 
beauty  of  the  preparation  to  be  photographed.  This 
must  be  so  arranged  that  it  shows  perfectly  clearly  all 
characteristic  parts ;  all  disturbing  accessories,  dust  and 
so  on,  must  be  removed,  for  they  are  equally  magnified 
with  the  object.  A  skilful  preparer  is  therefore  required 
to  do  anything  good  in  micro-photography,  which  also 
depends  on  the  excellence  of  the  instrument,  its  proper 


THE   APPLICATIONS   OF   PHOTOGRAPHY.  209 

arrangement,  and  the  choice  of  the  proper  time.  It  is 
important  in  an  instrument  to  correct  the  lenses  for 
chemically  operative  rays.  (See  p.  190.) 

Excellent  results  have  been  achieved  in  micro-photo- 
graphy by  Neyt  at  Ghent,  Girard  and  Lackerbauer  in 
Paris,  Fritzsch  and  Kellner  at  Berlin,  and  Woodward  in 
America.* 

Microscopic  photography  is  of  extraordinary  use  in 
anatomical  preparations,  which  quickly  change  and 
become  decomposed  in  chemical  combinations.  It  is 
also  useful  in  permanent  bodies ;  thus,  for  the  knowledge 
of  microscopic  crystals,  which  are  enclosed  in  many  kinds 
of  stones,  and  show  themselves  clearly  in  thinly  polished 
plates.  In  the  images  of  these  crystals,  the  angles  can 
be  easily  measured  with  the  help  of  a  protractor,  and 
from  them  the  nature  of  the  crystals  may  be  inferred. 
Professor  Gustavus  Eose  has  reproduced  in  steel  engrav- 
ings many  micro-photographs  of  this  kind,  taken  by  the 
author,  in  his  treatise  on  meteorites. 

SECTION  VIII. 
MICROSCOPIC  PHOTOGRAPHS  AND  THE  PHOTOGRAPHIC  PIGEON  POST. 

Nature  of  Microscopic  Photographs — Their  Importance  for  Libraries — 
Employment  of  the  Pigeon  Post. 

Some  years  ago  jewellery  and  toys  were  offered  for 
sale  in  Paris,  containing  small  magnifying  glasses.  If 
these  were  held  before  the  eye,  small  transparent  images, 
some  of  them  portraits,  and  others  writings,  came  into 

*  Full  details  are  given  in  "  Die  Photographic,  als  Hulfsmittel  mikro- 
skopischer  Forschung,"  by  Moitessier  and  Bennecke.  Brunswick: 
Vieweg. 


210  THE.  CHEMISTRY   OF   LIGHT. 

view.  These  little  images  were  what  were  called  a  micro- 
scopic photograph  on  glass.  Such  an  image  is  by  no 
means  the  representation  of  a  microscopic  object,  but  of 
a  large-sized  object,  only  the  image  is  so  small  that  a 
microscopic  lens  is  required  to  view  it.  The  production 
of  these  images  does  not  differ  from  that  of  others ;  it 
only  requires  an  instrument  that  projects  images  of 
microscopic  minuteness  in  an  optical  manner,  and  this 
is  effected  by  employing  small  lenses  of  very  short  focal 
distance.  In  using  these  a  direct  view  of  nature  is  not 
taken,  but  in  the  first  place  a  photographic  negative  is 
prepared  with  the  ordinary  camera  from  the  object 
chosen ;  after  this,  with  the  help  of  the  small  lenses,  little 
microscopic  images  on  glass  are  obtained  with  the 
ordinary  collodion  process.  These  little  glass  images 
are  polished  down,  a  small  lens  is  fastened  on  them,  and 
then  they  are  set  in  metal.  Such  images  are  in  them- 
selves little  else  than  toys,  which  can  even  be  perverted, 
if,  as  has  happened,  immodest  subjects,  taken  in  this 
fashion,  are  given  into  the  hands  of  unsuspecting  persons 
— a  fact  that  speedily  brought  this  branch  of  photography 
into  discredit.  But  there  are  circumstances  in  which 
such  microscopic  photographs  can  be  of  extraordinary 
value.  Simpson  in  England  has  called  attention  to  the 
fact  that,  by  the  help  of  photography,  the  contents  of 
whole  folios  can  be  concentrated  within  a  few  square 
inches,  and  that  the  substance  of  books  filling  entire 
halls,  when  reduced  by  microscopic  photography,  can  be 
brought  within  the  compass  of  a  single  drawer — a 
circumstance  which,  with  the  enormous  increase  of 
material  that  has  to  be  swallowed  by  our  libraries,  may 
be  of  importance.  No  doubt,  to  read  such  microscopic 


THE    APPLICATIONS    OF   PHOTOGRAPHY.  211 

works    requires   either  a   microscope   or   an  enlarging 
magic  lantern. 

Hitherto  it  has  not  been  applied  to  this  purpose, 
though  Scamoni's  heliographic  process,  described  further 
on,  would  considerably  facilitate  the  creation  of  such 
microscopic  libraries.  But  such  microscopic  photo- 
graphs have  obtained  great  importance  in  promoting 
pigeon  despatches.  During  the  siege  of  Paris  in  1870, 
the  blockaded  city  held  communication  with  the  world 
outside  by  means  of  balloons  and  carrier  pigeons.  The 
first  mode  of  communication  was  almost  engrossed  for 
political  objects ;  the  second  only  admitted  the  transmis- 
sion of  very  minute  writing.  Letters,  however  con- 
densed, could  scarcely  have  been  sent  more  than  two  or 
three  at  a  time  by  a  pigeon.  In  this  case,  microscopic 
photography  presented  a  valuable  means  of  concen- 
trating many  pages  on  a  collodion  film  of  only  one  square 
inch,  and  of  expediting  more  than  a  dozen  of  such 
almost  imponderable  films  packed  in  one  quill.  Dagrand 
at  Paris,  who  first  prepared  microscopic  photographs, 
also  set  going  the  system  of  these  pigeon  despatches. 
All  the  correspondence  which  had  to  be  diminished 
was  first  set  up  in  type,  and  printed  together  on  a  folio 
page.  A  microscopic  photograph  was  made  of  this  folio 
page,  contained  in  about  the  space  of  1J  square  inches. 
This  collodion  film,  with  the  image  upon  it,  was  then 
glazed  over  by  pouring  leather  collodion  over  it ;  that  is, 
collodion  containing  a  solution  of  glycerine.  This 
glucose  collodion  easily  dries,  separates  from  the  picture, 
and  forms  a  transparent  film.  A  membrane  of  this  kind 
could  contain  as  many  as  fifteen  hundred  despatches. 
At  the  place  of  arrival  these  membranes  were  unrolled, 


212  THE    CHEMISTRY   OF   LIGHT. 

-  *~    "    .'< 

and  then  enlarged  by  the  help  of  a  magic  lantern;  a 
number  of  writers  thereupon  set  to  work  to  copy  the 
enlarged  despatches,  and  ultimately  forwarded  them  to 
their  respective  addresses.  Thus  Paris  corresponded, 
by  the  aid  of  photography,  for  six  months  with  the 
world  without,  and  even  poor  persons  were  able  to  let 
their  relatives  know  that  they  still  lived. 

SECTION  IX. — PYRO.PHOTOGRAPHY. 

Fireproof  Views — Their  Production  by  Photography—  Grime's  Method — 
Its  Application  for  the  Decoration  of  Glass  and  Porcelain. 

An  ordinary  photograph  is,  as  paper,  very  combustible, 
and  exposed  to  injury  from,  corrosive  substances. 
Encaustic  images  on  porcelain  and  glarss  do  not  par- 
ticipate in  this  exposure  to  injury,  and  therefore 
attempts  have  been  made  to  prepare  fireproof  pho- 
tographs, especially  for  the  decoration  of  glass  and 
porcelain.  Success  has  crowned  these  efforts  in 
several  cases.  One  of  the  simplest  processes  in  that 
of  W.  Grime  at  Berlin. 

Griine  found  that  the  collodion  image — which,  as  we 
have  seen  (p.  112)  consists  of  minute  parts  of  silver — is 
capable  of  manifold  changes,  and  that,  moreover,  it  is 
easily  transferable,  with  its  elastic  collodion  film,  to  other 
bodies.  The  film,  with  the  picture,  can  be  placed  in 
different  solutions,  and  then  transferred  to  curved 
surfaces,  etc.  If  the  little  collodion  image  is  placed 
in  a  metal  solution,  a  chemical  change  ensues.  As- 
suming the  metal  solution  to  contain  chloride  of  gold, 
then  the  chlorine  passes  over  to  the  silver,  of  which 
the  picture  consists,  chloride  of  silver  is  formed,  and 


^^ 

(UK  '?] 

THE   APPLICATIONS^?   PHOTOGPtAPHY.        I/       213 

metallic  gold  is  precipitateo^^ft^l^sdaM^powder  on 
the  outline  of  the  picture.  Thus  a  gold  picture  is 
obtained. 

With  certain  precautions  this  can  be  transferred  to 
and  made  encaustic  on  porcelain.  By  this  means  an 
unpolished  image  is  obtained,  which  can  be  rendered 
brilliant  by  polishing.  Griine  has  employed  this  to  pro- 
duce gold  ornaments  on  glass  and  porcelain.  Drawings 
and  patterns  of  different  kinds  are  photographed;  the 
image  obtained  is  changed  into  one  of  gold,  then  burnt 
in,  and  thus  the  most  beautiful  and  complicated  decora- 
tions can  be  produced  without  the  assistance  of  the 
porcelain  painter. 

If  a  silver  picture  be  plunged  into  a  solution  of 
platinum  instead  of  a  solution  of  gold,  a  platinum  image 
is  obtained.  This  assumes  a  black  colour  on  being 
burnt  into  the  porcelain.  In  this  manner  black  portraits, 
landscapes,  etc.,  have  been  produced  on  porcelain. 

Images  of  this  kind  in  other  tints  can  be  represented 
as  black.  For  example,  if  the  image  is  dipped  in  a 
combined  solution  of  gold  and  platinum,  the  gold  and 
platinum  are  precipitated  on  the  picture.  The  image 
thus  obtained,  if  burnt  in,  presents  a  very  agreeable 
violet  tint. 

Solutions  of  uranium,  of  iron,  and  of  manganese  effect 
precipitates  on  a  collodion  picture,  modifying  its  colour, 
and,  when  burnt  in,  producing  different  brownish  or 
blackish  tints.  We  shall  see,  later  on,  that  there  are 
other  means  of  producing  such  pyro-photographs. 
Details  will  be  found  in  the  chapter  on  the  photo- 
chemistry of  chromic  combinations. 


214  THE    CHEMISTRY   OF   LIGHT. 

SECTION  X. — MAGIC  PHOTOGRAPHY. 
Invisible  Photographs — Magic  Pictures  and  Magic  Cigar  Ends. 

Closely  connected  with  Grime's  process  for  producing 
porcelain  pictures  is  what  is  called  magic  photography. 
A  few  years  ago  small  wrhite  sheets  of  paper  were 
offered  for  sale  which,  on  being  covered  with  blotting- 
paper  and  sprinkled  with  water,  displayed  an  image  as 
if  by  magic.  The  white  sheets  of  paper,  to  all  appear- 
ance a  blank,  were  photographs  which  had  been 
bleached  by  plunging  in  chloride  of  mercury.  If  a 
photograph  not  containing  gold — all  the  usual  paper 
photographs  contain  gold — be  plunged  in  a  solution  of 
chloride  of  mercury,  a  part  of  the  chlorine  passes  over 
to  the  silver  of  the  picture,  and  changes  this  brown 
mass  into  white  chloride  of  silver,  which  is  invisible  on 
the  white  paper.  At  the  same  time  a  chloride  of 
mercury  containing  less  chlorine, — hypo-chloride  of 
mercury, — which  is  also  white,  and  therefore  invisible 
on  the  white  paper,  is  precipitated.  Now,  there  are 
different  substances  which  colour  black  this  white 
hypo-chloride  of  mercury.  Among  these  are  hypo- 
sulphite of  soda  and  ammonia.  If,  therefore,  the 
invisible  picture  is  moistened  with  one  of  these  sub- 
stances, it  is  coloured  black  and  becomes  visible.  In  the 
magic  photographs  formerly  sold  there  was  hypo-sulphite 
of  sodium  in  the  blotting  paper ;  this  became  dissolved 
on  moistening  the  paper,  or  water  penetrated  to  the 
magic  image  lying  under  it  and  made  it  visible. 

Quite  a  different  kind  of  magic  photograph  was 
offered  for  sale  some  years  later — the  magic  cigar  tips. 
These  contained  a  small  sheet  of  paper  between  the 


THE    APPLICATIONS   OF   PHOTOGRAPHY.  215 

cigar  and  the  mouthpiece,  which  the  cigar  smoke 
penetrates ;  with  continued  smoking  an  image  became 
visible  on  the  sheet  of  paper,  which  contained  a  magic 
photograph  of  the  kind  described  above.  The  image 
was  brought  out  by  the  vapour  of  ammonia  which  is  in 
cigar  smoke,  and  which  has  also  the  property  of 
colouring  black  the  magic  photographs. 

The  magic  photographs  of  recent  times  were  intro- 
duced at  Berlin  by  Grime,  but  their  principle  was  known 
before,  as  J.  Herschel  had  produced  some  in  1840. 

SECTION  XI. — SCAMONI'S  HELIOGRAPHIC  PROCESS. 

Defects  of  the  Positive  Silver  Process — Advantages  of  the  Typographic 
Press — Belief  of  the  Photographic  Negative — Impress  of  the  Same 
on  Copper. 

It  was  stated  at  an  earlier  page,  that  the  positive 
photographic  process  had  the  defect  of  working  very 
slowly.  Every  picture  that  has  to  be  copied  from  a 
negative  must  have  a  longer  or  shorter  exposure  to  the 
light.  Hence,  the  worse  the  light,  the  longer  the  time 
required.  This  is  of  no  moment  with  a  dozen  portraits, 
but  if  hundreds  or  thousands  are  to  be  prepared,  time  is 
of  consequence. 

Another  disadvantage  in  the  silver  copy  is  its  high  price 
and  doubtful  durability.  Attempts  have  been  made,  since 
the  discovery  of  photography,  to  overcome  these  defects 
by  combining  it  with  printing  press  processes — litho- 
graphy or  metal-type  press.  To  carry  out  the  metal 
press,  an  engraved  plate  is  used ;  that  is,  a  metal  plate  in 
which  the  drawing  is  deeply  incised.  This  is  covered  with 
engraver's  ink,  the  ink  penetrates  into  the  incisions,  and 
under  a  heavy  press  a  steel  engraving  or  copper-plate 


216  THE    CHEMISTRY   OF   LIGHT. 

engraving  is  thus  produced.  Impressions  of  this  kind 
can  be  made  in  a  short  time  in  great  quantities,  without 
the  help  of  light,  and  without  employing  expensive  salts 
of  silver.  We  showed  in  the  first  chapter  (p.  10)  that  an 
incised  drawing  on  a  metal  plate  can  be  made  with  the 
help  of  photography.  We  mentioned  asphaltum  as  an 
auxiliary  to  this  end.  But  the  same  object  can  be 
attained  in  another  way ;  and  one  of  the  most  original 
is  that  of  Herr  G.  Scamoni,  at  St.  Petersburg,  the  able 
heliographer  of  the  Imperial  Eussian  expedition  for 
procuring  State  papers. 

He  observed  that  an  ordinary  photographic  negative 
does  not  form  a  plane  surface,  but  appears  in  relief;  the 
transparent  places — shadows — being  in  basso,  and  the 
light  in  alto  relievo.  But  this  relief  is  very  faint. 
Scamoni  tried  to  increase  it  by  treating  the  freshly  taken 
and  developed  picture  with  pyrogallic  acid  and  solution 
of  silver.  In  this  manner  fresh  silver  powder  was 
precipitated  on  the  picture,  which  has  the  property  of 
attracting  and  retaining  silver  separated  chemically. 
The  relief  was  considerably  increased  by  this  strengthen- 
ing process.  It  can  be  augmented  by  a  treatment  with 
chloride  of  mercury  and  iodide  of  potassium,  which 
conduct  the  metal  silver  of  the  picture  into  more  solid 
combinations.  A  relief  was  ultimately  obtained  nearly 
as  high  as  that  of  the  incisions  of  an  engraved  copper- 
plate. When  a  linear  drawing  has  been  taken  in  this 
way,  and  after  the  negative  has  been  obtained,  a  positive 
has  been  prepared  by  repeating  the  collodion  process  in 
the  camera,  and  the  latter  has  been  brought  out  enough 
in  high  relief  by  strengthening,  all  the  means  are  in  hand 
to  produce  an  engraved  copper -plate  from  the  image 


THE   APPLICATIONS   OF   PHOTOGRAPHY.  217 

thus  received.  The  relief-like  photographic  plate  is 
brought  into  a  galvano-plastic  apparatus,  of  which  we 
shall  speak  farther  on.  This  produces  on  the  plate  a 
connected  copper  precipitate,  which  is  in  basso  or  low 
relief  where  the  plate  shows  high  relief;  that  is,  where 
there  are  strokes  or  outlines.  Thus  a  copper-plate  is 
obtained  from  which  impressions  can  be  taken  as  well 
as  from  one  that  has  been  engraved.  This  process  is 
now  used  to  reproduce  drawings  like  copper  plate. 

Maps  are  prepared  in  this  manner,  in  which  the 
drawing  can  be  photographically  enlarged  or  diminished ; 
also  writings  on  an  enlarged  and  diminished  scale. 
Scamoni  has  thus  reduced  all  the  manifestoes  of  the 
Emperor  Alexander,  as  also  a  page  of  the  illustrated 
journal  "  Ueber  Land  und  Meer  "  on  leaves  of  one  inch 
in  width,  on  which  the  writing  is  perfectly  legible 
through  the  microscope.  Helps  of  this  kind  are  not 
mere  play,  but  they  have  a  great  importance  for  the 
preparation  of  paper  money  and  for  libraries,  as  we 
showed  at  pp.  11  and  210. 

SECTION  XII. — PHOTOGRAPHY  AND  JURISPRUDENCE. 

Photographic    Authentication    Cards — Photographs     of     Criminals,    of 
Eailwaj  Accidents,  Fires,  Documents,  etc. 

The  application  of  photography  to  jurisprudence  is 
of  great  interest.  The  faithful  likeness  of  a  man,  or  of 
an  object,  makes  their  recognition  more  certain  than  the 
most  circumstantial  description  in  words;  and  photo- 
graphy gives  us  such.  Accordingly,  repeated  attempts 
have  been  made  to  utilize  it  as  a  means  of  authentica- 
tion. This  was  first  attempted  in  1865,  when  the 


218  THE    CHEMISTKY   OF    LIGHT. 

season  tickets  for  the  photographic  exhibition  at  Berlin 
contained  the  portrait  of  the  holder,  that  they  might  not 
be  transferred.  This  plan  is  now  adopted  in  the  season 
tickets  of  the  Zoological  Gardens  of  Berlin.  It  is  still 
more  important  for  the  recognition  of  criminals.  Persons 
subject  to  various  penalties  are  now  photographed  in 
prisons,  partly  as  a  means  of  recapture  in  case  of  evasion, 
partly  to  detect  them  in  case  they  should  be  again 
brought  in  under  a  false  name. 

Justizrath  Odebrecht,  in  a  treatise  on  jurisprudence, 
recommends  the  taking  of  the  bodies  found,  and  in  case 
of  murder  that  of  the  victim  and  the  surroundings,  for 
the  information  of  judges*.  This  has  been  repeatedly 
done.  Further,  railway  trains  that  have  suffered  an 
accident,  buildings  that  have  been  destroyed  by  fire  or 
the  elements,  are  photographed  for  the  information  of 
railway  and  insurance  companies,  or  of  the  legal 
authorities.  Photography  is  very  advantageous  in  this 
matter,  through  the  rapidity  of  its  operation,  which  can 
be  completed  within  a  few  minutes,  and  carried  on 
even  during  the  restoration  of  the  building.  It  is  also 
of  value  in  jurisprudence,  by  detecting  forgeries.  Very 
frequently  forged  cheques  are  photographed  in  order  to 
send  a  copy  for  the  information  of  those  interested. 
Stolen  and  recovered  articles  are  also  often  photo- 
graphed to  bring  them  to  the  notice  of  the  proprietor. 
In  many  large  cities  the  police  cause  pickpockets  and 
sharpers  to  be  photographed,  and  show  an  album  of 
this  kind  to  persons  who  have  been  robbed. 


THE   APPLICATIONS   OF   PHOTOGRAPHY.  219 

SECTION  XIII. —PHOTOGRAPHY,  INDUSTRY,  AND  ART. 

Photography  as  a  Means  of  Artistic  Culture — Extension  of  the  Art  of 
Drawing  through  Photography-— Pattern  Cards — Application  of  it  to 
Building  Plans — Estimation  of  Solids  by  Photography. 

We  have  already  laid  stress  upon  the  importance  of 
photography  in  works  of  art.  It  makes  every  work  of 
art  accessible  to  persons  of  slender  means,  and  therefore 
it  has  become  as  important  an  auxiliary  for  popular 
culture  in  the  province  of  art  as  the  printing  press  is  for 
science. 

Photography  is  equally  important  in  those  branches 
of  industry  in  which  graphic  representations  are  indis- 
pensable; for  example,  architecture  and  the  construc- 
tion of  machinery.  In  their  case  photography  forms 
an  enlargement  of  the  art  of  drawing,  effecting  in  a  few 
minutes  what  the  draughtsman  could  only  accomplish 
in  several  hours  or  days,  and  representing  with  a  faith- 
fulness to  which  no  draughtsman  could  attain.  In 
this  connection  we  have  already  described,  in  our  second 
chapter,  the  technical  importance  of  the  licht-paus 
process.  It  is  the  easiest  kind  of  photography,  but  it 
only  gives  copies  of  the  size  of  the  original ;  however,  the 
negative  process  allows  an  enlarged  or  diminished  copy 
to  be  taken  of  every  drawing,  according  to  option. 
Photography  is  already  very  generally  used  for  these 
reproductions.  It  is  equally  important  -for  taking  views 
direct  from  nature,  be  they  machines  or  parts  of 
machines,  buildings  or  parts  of  buildings.  Pictures  of 
this  kind  present  not  only  a  graphic  image,  but  they 
serve  for  instruction  and  demonstration  in  lectures. 
Nay,  when  a  house  is  to  be  photographed,  and  measures 


220  THE    CHEMISTRY   OF   LIGHT. 

have  been  laid  clown  for  its  length,  breadth,  and  depth, 
the  dimensions  of  the  particular  part  can  be  taken 
from  the  photograph  by  making  allowance  for  the 
foreshortenings  in  perspective.  Pictures  on  a  small 
scale  are  commonly  sent  out  as  specimen  cards.  Iron 
foundries,  manufactories  of  bronzes  and  of  porcelain 
frequently  issue  lists  of  prices  with  photographic  illus- 
trations, of  which  the  images  are  multiplied  from 
negatives  of  originals  by  the  printing  of  light.  (See  the 
following  chapter.) 

Further,  the  original  application  of  photography  is 
that  relating  to  the  plans  of  buildings.  Architects  who 
are  at  a  distance  from  a  building  under  their  direction 
cause  photographs  to  be  taken  every  week,  giving  them 
a  clear  picture  of  the  progress  of  the  building.  We 
have  already  hinted  at  the  services  that  photography 
can  render  in  the  manufacture  of  porcelain,  and  further 
in  combination  with  the  multiplying  arts.  We  shall  learn 
more  on  this  subject  in  the  following  chapter. 


CHAPTEE  XV. 

CHROMOPHOTOGKAPHY. 

WE  have  given  a  full  account,  in  the  first  part  of  our 
book,  of  the  chemical  and  physical  principles  of  photo- 
graphy with  salts  of  silver,  and  of  its  application  to  art, 
science,  life,  and  industry. 

Numerous  attempts  have  been  made  to  substitute 
other  sensitive  materials  for  the  expensive  salts  of  silver, 
and  some  of  these  attempts  have  been  crowned  with 
success.  It  is  true  that  no  substance  has  been  hitherto 
found  permitting  a  negative  to  be  prepared  in  the 
camera  as  easily  as  iodide  of  silver.  For  the  production 
of  camera  pictures  from  nature  we  are  exclusively  con- 
fined to  iodide  of  silver  and  bromide  of  silver.  But  the 
case  is  different  with  the  production  of  positives  from 
negatives  already  existing.  These  can  be  successfully 
produced,  not  only  by  the  help  of  salts  of  silver,  but 
also  of  other  metallic  combinations.  I 'admit  that  the 
results  obtained  are  inferior  in  beauty  to  the  silver 
pictures,  but  we  shall  see,  later  on,  that  they  admit  of 
multiplication  through  combination  with  printing  by 
impression  without  the  help  of  light.  We  shall  now 
describe  the  most  important  of  these  processes. 


222  THE    CHEMISTRY   OF    LIGHT. 

SECTION  I. — CHROMIC  COMBINATIONS. 

Oxides — Combinations  of  Chromium  with  Oxygen — Oxide  Salts  of 
Chromium — Protoxide  and  other  oxides  of  Chromium — Chromic  Acids 
— Chromic  Acid  Salts  in  the  Light — Ponton's  Discoveries. 

A  black  mineral  called  chrome  iron  ore  occurs  in 
nature,  especially  in  Sweden  and  America.  If  this 
be  dissolved  with  carbonate  of  potash,  a  beautiful 
orange-red  salt  is  formed,  which  dissolves  in  water  and 
easily  crystallizes  on  evaporation.  This  orange-red  salt  is 
the  bichromate  of  potash.  It  consists,  as  implied  by 
the  name,  of  chromic  acid  and  potassium.  The  latter  is 
the  chief  component  part  of  our  potash;  the  former 
consists  of  a  metal,  like  iron,  and  of  oxygen.  Chromium 
and  oxygen  form  together  very  different  combinations. 

28  parts  chromium  with  8  parts  oxygen  to  protoxide  of  chromium. 
28      „  „  „    12      „  „         „  sesquioxide  of  chromium. 

28      „  „  „    16      „  „         „  suboxide  of  chromium. 

28      „  „  „   24      „          „         „  chromic  acid. 

The  last  combination,  chromic  acid,  is  the  best  known 
of  all ;  on  adding  to  it  sulphuric  acid,  it  changes  to 
chromate  of  potash,  and  crystallizes  into  red  needles, 
which  easily  lose  part  of  its  oxygen.  For  example,  if 
chromic  acid  is  dropped  upon  alcohol,  the  latter  becomes 
inflamed,  because  it  immediately  withdraws  oxygen 
from  the  chromic  acid  and  changes  it  into  a  green  body, 
the  oxide  of  chromium.  The  oxide  of  chromium  forms 
salts  with  acids;  for  example,  sulphate  of  chromium. 
This  unites  again  readily  with  sulphate  of  potash  to 
form  a  dry  salt,  which  is  known  by  the  name  of  chrome 
alum,  and  is  sold  crystallized  in  very  beautiful  dark 
violet  octahedra.  It  is  employed  in  painting  and  dye- 
ing, together  with  chromate  of  potash. 


CHROMO  -PHOTOGRAPHY. 


223 


If  chromate  of  potash  be  mixed  with  a  solution  of 
green  vitriol,  the  green  vitriol  takes  up  a  part  of  the 
oxygen  of  the  chromate  of  potash,  and  a  brown 
protoxide  of  chromium  is  precipitated.  This  is  often 
formed  by  the  operation  of  substances  absorbing  oxygen 
on  chromic  acid  or  its  salts. 

Chromic  acid  is  of  special  interest  in  the  object  that 
engages  it,  because  both  it  and  its  salts  are  sensitive  to 
light.  Pure  chromic  acid,  and  also  chromate  of  potash, 
do  not  change  in  the  light;  they  can  be  exposed 
for  years  to  the  sunlight  without  any  decomposition 
being  perceived.  As  soon  as  a 
body  is  present  that  can  be  united 
with  oxygen — for  example,  wood- 
fibre,  paper,  etc. — the  light  imme- 
diately produces  its  effect.  This 
fact  was  published  in  the  year 
of  the  discovery  of  photography, 
1839,  by  Mungo  Ponton,  and  in 
the  "New Philosophical  Journal " 
he  writes  : — 

"  If  paper  is  saturated  with  a  solution  of  chromate  of 
potash,  it  becomes  sensitive  to  the  sun's  rays.  If  an 
object  be  placed  upon  it,  the  part  exposed  to  the  light 
quickly  assumes  a  yellowish -brown  tint,  shading  more 
or  less  into  orange,  according  to  the  strength  of  the 
light.  The  part  covered  by  the  object  retains  its 
original  clear  yellow  colour,  and  the  object  is  imprinted 
as  a  clear  outline  on  a  dark  ground,  with  different 
shades  of  colour,  varying  with  the  different  degrees  of 
transparency  of  different  parts  of  the  object.  In  this 
condition  the  picture,  though  very  beautiful,  is  not 


Fig.  89. 


224  ^    THE    CHEMISTKY   OF    LIGHT. 

lasting ;  to  fix  it,  it  is  sufficient  to  plunge  it  in  water, 
whereupon  all  parts  of  the  salt  that  were  not  touched 
by  the  light  are  quickly  dissolved,  while  those  on  which 
the  light  could  operate  are  perfectly  fixed  on  the  paper. 
By  the  last  process  a  white  picture  is  obtained  on  an 
orange  ground,  and  it  is  perfectly  durable.  If  it  be 
exposed  for  many  hours,  the  ground  tint  loses  intensity 
of  colour,  but  not  more  than  happens  with  other 
coloured  matters." 

It  appears  that  Mungo  Ponton  made  experiments, 
like  Talbot,  in  the  first  period  of  silver  photography. 
Perhaps  he  also  copied  leaves  (see  p.  5).  The  copies 
which  are  produced  in  the  above  manner  on  chromate 
of  potash  are,  however,  immeasurably  fainter  than  the 
copies  on  silvered  paper. 

They  can  at  any  time  be  easily  prepared,  by  plunging 
a  piece  of  white  paper  into  a  solution  of  chromate  of 
potassium  in  the  dark,  by  the  light  of  a  lamp.  After 
a  minute,  they  are  taken  out  and  suffered  to  dry.  It 
is  best  to  suspend  them,  and  the  dried  surface  is 
exposed  to  the  light  in  a  copper  frame  under  dried 
leaves,  or  a  drawing,  or  a  negative. 

The  chromic  acid  is  then  reduced  to  brown  protoxide 
of  chromium,  but  if  the  exposure  lasts  very  long  the 
reducing  process  goes  further,  and  a  green  oxide  of 
chromium  is  formed.  In  this  case  the  picture  appears 
fainter. 

Accordingly,  Ponton's  experiment  remained  a  mere 
curiosity  until  the  inventor  of  photography  on  silvered 
paper  discovered  another  property  of  chromate  of 
potash,  which  led  to  the  most  extensive  applications. 

This  property  consists  in  the  operation  of  the  com- 
binations of  chromium  on  glue. 


225 


SECTION  II. — HELIOGRAPHY  WITH  SALTS  OF  CHROMIUM. 

Properties  of  Gelatine— Chromate  of  Potassium  and  Glue — Talbot's  Dis- 
covery— Effect  of  Light  on  the  Solubility  of  Glue — Photographic 
Steel  Engraving — Pretsch's  Photogalvanography — Printing  in  High 
and  Low  Eelief — Importance  of  the  Former— Difficulty  of  producing 
Half -Tones  by  the  Heliographic  Method. 

Glue  in  its  purest  form,  known  by  the  name  of  gelatine, 
is  insoluble  in  cold  water,  but  it  sucks  up  cold  water 
like  a  sponge,  and  thereby  swells.  If  it  is  warmed  with 
water,  it  dissolves ;  but  on  cooling  the  solution  hardens 
to  a  jelly.  This  property  is  used  to  thicken  soups.  If 
to  the  warmed  solution  of  glue  be  added  aluminum,  or 
a  salt  of  the  oxide  of  chromium,  or  chrome  alum,  the 
glue  becomes  insoluble  in  water,  and  forms  a  precipitate. 
On  this  is  based  the  well-known  system  of  white  tanning; 
for  in  the  tanning  of  a  piece  of  leather  the  aluminum 
combines  with  the  gelatine  contained  in  the  leather, — 
chondrin, — and  this  becomes  thereby  insoluble,  and  at 
the  same  time  durable. 

Chromate  of  potash  and  glue  can  be  dissolved  together 
in  warm  water  in  the  dark,  without  the  glue  suffering 
from  the  chromic  salt.  If  a  plate  or  a  sheet  of  paper 
be  covered  with  a  solution  of  chromate  of  potash  and 
the  film  be  allowed  to  dry,  it  becomes  firm,  and  yet 
remains  soluble  in  water  as  long  as  it  is  kept  in  the 
dark.  But  as  soon  as  the  film  is  influenced  by  the  light, 
the  chromate  of  potash  is  reduced  to  oxide  of  chromium, 
and  this  tans  the  film  of  gelatine;  that  is,  makes  it 
insoluble  in  water. 

This  observation  was  made  by  Pox  Talbot  in  1852,  and, 
as  a  careful  observer,  he  knew  directly  how  to  turn  it  to 


226  THE    CHEMISTRY   OF   LIGHT. 

account.  He  coated  a  steel  plate  with  a  solution  of  chro- 
mium and  gelatine,  let  it  dry  in  the  dark,  and  then  exposed 
it  under  a  drawing  or  a  positive  glass  picture.  The  black 
lines  kept  back  the  light.  Accordingly,  at  these  places  the 
gelatine  remained  soluble,  but  it  became  insoluble  at  the 
white  places,  through  the  operation  of  light.  After  the 
exposure  he  washed  the  plate  in  the  dark  with  warm 
water.  By  this  means  the  places  that  had  remained 
soluble  under  the  black  lines  became  dissolved;  the 
others  wrere  retained  on  the  plate.  Thus  Talbot 
obtained  a  drawing  on  the  metal  itself  on  a  brown 
ground.  This  is  worthless  by  itself,  but  it  provides  the 
means  of  producing  a  steel  plate  for  engraving. 

We  have  already  explained,  at  p.  215,  the  nature  of 
steel  engraying  and.  copper-plate  engraving.  Both 
processes  consist  in  the  production  of  a  metal  plate 
which  contains,  in  incised  lines,  the  drawing  that  is  to 
be  reproduced.  These  lines  become  coloured  when 
engravers  are  used,  and  imprint  it  upon  the  paper.  The 
hard  steel  plates  have  the  advantage  of  lasting  for  many 
more  copies  than  the  softer  copper  plate ;  only  the  steel 
engravings  are  far  inferior  to  copper-plate  in  artistic 
beauty,  and  therefore  the  former  have  lost  favour.  But 
the  steel  engraving  is  very  important  to  prepare 
technical  and  scientific  diagrams,  paper  money,  and  the 
like,  as  less  artistic  beauty  is  required  in  their  case.  It 
was  steel  plates  of  this  kind  that  Talbot  produced  by 
the  help  of  light. 

We  have  seen  that  his  steel  plate  was  covered  with 
an  insoluble  film  of  gelatine,  and  that  the  metal  was 
uncovered  at  all  places  where  the  light  had  not 
operated.  He  poured  on  it  a  fluid  which  ate  into  the 


CHROMO-PHOTOGEAPHY.  227 

steel ;  for  example,  a  mixture  of  acetic  acid  and  nitric 
acid.  This  mixture,  of  course,  only  took  effect  where  the 
steel  was  exposed,  and  thus  produced  an  incised  drawing 
^  in  the  steel  plate,  so  that  the  latter,  after  being  cleaned, 
gives  as  good  an  engraving  as  if  it  were  the  work  of  the 
engraver. 

Thus  a  new  process  was  found  to  replace  the  difficult 
work  of  the  copper-plate  engraver  by  the  chemical 
operation  of  light. 

We  have  mentioned  in  the  first  chapter  a  similar 
process,  based  on  the  application  of  asphaltum ;  also  a 
different  one  by  Scamoni.  (See  p.  215.) 

This  discovery  of  Talbot  was  soon  followed  by  a  more 
productive  one  on  the  same  ground. 

An  Austrian,  Paul  Pretsch,  prepared,  in  1854,  coppei 
plates  by  a  similar  process,  with  the  help  of  galvano- 
plastic.  He  also  took  a  film  of  gelatine,  which  contained 
chromate  of  potash,  exposed  this  under  a  negative  or 
a  positive  picture,  and  then  washed  it  in  hot  water. 

After  doing  this  all  the  places  were  retained  which 
had  become  insoluble  through  the  light,  and  after  the 
washing  and  drying  they  stood  out  in  high  relief. 

Accordingly,  in  copying  under  a  positive,  the  lines 
which  were  black  in  the  original  appeared  in  low  relief, 
and  the  white  parts  in  high  relief. 

This  kind  of  film  in  relief  was  placed  in  a  galvano- 
plastic  apparatus.  This  apparatus  has  ihe  property  of 
precipitating  copper  or  other  metals  on  a  surface.  It 
consists  of  a  galvanic  element,  as  described  p.  71,  with 
whose  pole  a  trough  with  a  solution  of  copper  and  vitriol 
is  united.  To  the  zinc  pole  are  suspended,  by  means  of 
the  rod  B  (Fig.  90),  the  reliefs  which  it  is  desired  to 


228 


THE    CHEMISTRY   OF   LIGHT. 


imprint,  after  they  have  been  made  conductors  by  a 
coating  of  graphite ;  a  copper  plate  is  suspended  at  the 
copper  end  D.  As  soon  as  the  galvanic  stream  operates, 
the  fluid  is  decomposed.  The  copper  adheres  to  the 
relief,  and  the  thickness  of  the  copper  depends  on  the 
time  the  current  is  allowed  to  last.  Accordingly,  plates 
of  any  thickness  can  be  produced. 

If  the  original  form  was  in  low  relief,  the  galvano- 
plastic  impression  will  be  in  high  relief,  and  vice  versa. 
Therefore,  in  the  above  case  an  impression  is  received 
with  lines  in  high  relief. 


Fig.  90. 

This  kind  of  plate  is  also  adapted  to  give  impressions, 
but  rather  differently  from  an  incised  copper  plate. 

In  an  incised  copper  plate,  the  engraver's  ink  is 
rubbed  into  the  incised  marks,  and  then  under  strong 
pressure  conveyed  to  paper. 

In  a  plate  with  a  drawing  in  high  relief,  the  impres- 
sion takes  place  as  in  printing;  the  raised  places  are 
rubbed  over  with  printer's  ink  by  the  help  of  a  leather 
ball,  or  of  a  cylinder  blackened  with  ink,  and  then  im- 


CHROMO-PHOTOGRAPHY.  229 

printed  on  paper.  Letter-press  is  produced  in  this 
manner ;  all  its  letters  are  in  high  relief,  also  all  wood- 
cuts which  accompany  the  text. 

The  printing-press  is  the  simplest  and  cheapest  mode 
of  multiplying  copies.  It  admits  of  the  use  of  cheap 
papers,  whilst  copper-plate  engraving  requires  a  thick, 
soft,  special  paper.  The  printing-press,  moreover,  admits 
of  woodcuts  being  printed  in  the  text,  whilst  copper-plate 
printing  requires  special  tables.  Lastly,  the  printing- 
press  works  with  extreme  rapidity — steam  press — 
whereas  copper -plate  printing  requires  much  more  time. 

Further,  the  printing-press  does  not  use  up  the  type 
rapidly,  as  it  works  under  feeble  pressure ;  while  copper- 
plate printing,  which  requires  strong  pressure,  wears 
the  plate  considerably,  so  that  after  striking  off  a 
thousand  copies,  the  impressions  are  no  longer  as  good 
as  at  first. 

The  production  of  plates  is  very  important  for  the 
printing-press,  and  Pretsch  has  taken  the  lead  here. 
His  process  did  not,  indeed,  produce  the  most  perfect 
results.  The  low-relief  plate  which  he  produced  on  the 
gelatine  film  by  the  help  of  light  was  not  deep  enough 
to  produce  a  high  relief  with  galvanic  impression ;  but 
this  is  necessary,  for  otherwise  the  printer's  ink  pene- 
trates into  the  incised  parts,  which  ought  to  remain  white, 
while  the  washing  of  the  proposed  chromo-glucose  pictures 
with  hot  water  easily  dissolves  the  finer  particles  of  the 
picture,  and  this  detracts  materially  from  the  value  of 
the  copies.  Moreover,  the  printing  with  the  help  of 
galvano-plastic  has  its  difficulties.  The  film  of  gelatine 
liquifies  in  part  and  loses  its  form.  In  short,  the  affair  is 
not  so  simple  as  it  appears ;  little  difficulties  exist,  and 
11 


230  THE    CHEMISTBY   OF   LIGHT. 

these  occasion  errors  which  the  unprofessional  hardly 
observe,  but  which  considerably  diminish  the  effective- 
ness of  the  picture. 

At  an  early  period  it  was  found  that  these  processes 
offered  a  special  difficulty,  viz.  the  reproduction  of  the 
transitions  from  light  to  shade — the  half  tones.  These 
were  so  very  imperfect  that  the  representation  of  natural 
objects — portraits  and  landscapes— was  speedily  given  up, 
and  people  confined  themselves  to  representing  drawings, 
maps,  and  the  like,  on  an  enlarged  or  diminished  scale, 
and  thereby  to  producing  stereotype  plates  for  copper 
engraving  and  printing.  This  application  is  of  no  little 
importance,  for  it  prepares  a  metal  plate  for  printing,  by 
the  help  of  light,  in  as  many  hours  as  an  engraver 
requires  days,  and  at  far  less  cost. 

We  add  two  plates  to  the  present  work,  which  by  the 
help  of  gelatine  and  salts  of  chromium,  by  a  modifica- 
tion of  the  process  now  described,  have  been  carried  into 
effect  by  Scamoni  at  St.  Petersburg.  Both  are  impres- 
sions of  heliographic  plates :  the  smaller  one — Plate 
III.,  "Am  Bhein" — a  plate  in  high  relief,  which  is 
printed  in  the  letter-press  ;  the  other — Plate  IV., 
"  Johannisfest " — a  low  relief,  in  the  style  of  copper 
engraving. 

THE  SECTION  III. — THE  PRODUCTION  OF  PHOTO-RELIEFS. 

Photo-sculptures— The  Pantograph — The  Fount  Process — Chromogela- 
tine-relief— Fount-relief  by  Cold  Water— Belief  by  Cold  Water — 
Difficulty  of  its  Production — The  Transfer  Process. 

More  than  ten  years  ago  intelligence  was  received  from 
Paris  of  an  entirely  new  discovery — photo -sculpture — 
which  was  said  to  produce  statues  by  the  help  of  light. 


CHftOMO-PHOTOGRAPHY.  231 

According  to  the  description,  this  was  effected  by  a 
circuitous  process :  a  person  was  placed  in  the  middle  of 
a  circle,  and  around  him  were  placed  about  twenty  photo- 
graphic apparatus,  which  at  a  given  moment  took 
twenty  pictures  of  the  person,  and  represented  him  on 
every  side.  These  photographs  were  afterwards  trans- 
ferred with  their  outlines  to  clay,  by  means  of  an  instru- 
ment commonly  called  a  pantograph.  This  consists  of 
a  system  of  bars  a  led  (Fig.  91).  Of  this  system  one 
bar  is  placed  at  a  fixed  point  x,  the  others  are  movable 
at  the  joints ;  m  n  are  two  pegs.  If  one  peg  m  is  carried 


Fig.  91. 

along  a  drawing,  the  other  peg  n  makes  the  same  move- 
ment, and,  if  a  piece  of  paper  be  placed  under  the  peg 
n,  draws  exactly  the  same  line  which  the  first  peg  m 
describes.  If  instead  of  the  peg  n  we  conceive  a  knife, 
which  cuts  out  in  clay  the  outline  described  by  the  first 
peg  m,  a  profile  is  obtained  in  clay  by  m6ving  the  peg 
m  along  the  circumference  of  an  image,  and  in  this 
manner  all  the  outlines  of  the  person  taken  can  be 
transferred  to  clay.  This  photo-sculpture,  as  it  is  called, 
can  only  be  carried  out  imperfectly.  A  careful  manipu- 
lation by  a  very  clever  artistic  hand  is  necessary  for  the 


232  THE    CHEMISTRY   OF   LIGHT. 

work,  and  is  indeed  the  essential  matter.  As  far  as  the 
author  has  examined  the  matter,  the  pantograph  is  a 
mere  pretence.  A  clever  artist  models  the  bust  accord- 
ing to  the  photograph  at  hand. 

Nevertheless,  there  are  reliefs  produced  by  light,  and 
these  reliefs  are  not  inventions  of  advertisers ;  they  are 
easily  produced,  and  it  is  surprising  that  the  process  has 
not  yet  made  a  stand. 

We  have  explained  above  the  properties  of  gelatine, 
and  remarked  that  it  has  the  capacity  of  liquifying  in 
cold  water.  This  property  is  lost  if  the  gelatine  is 
saturated  with  chromate  of  potash,  and  exposed  to  the 
light.  If  this  exposure  is  made  under  a  negative,  all 
the  places  situated  under  the  transparent  parts  lose 
their  capacity  of  liquifying,  while  the  other  places  not 
affected  by  the  light  retain  it.  Accordingly,  if  the  ex- 
posed film  be  thrown  into  water,  the  places  which  are 
not  affected  by  light  swell,  whilst  those  affected  by  light 
remain  in  low  relief.  The  result  is  a  true  relief, — the 
lights  are  in  high  relief,  the  shadows  are  in  low  relief, 
and  this  is  so  strong  that  it  can  be  cast  in  gypsum.  To 

this  end  the  relief  is  dried  with 

^ I  blotting-paper,  rubbed  with  oil, 

and  then  a  paste  of  gypsum 
is  poured  over  it.  This  soon 


hardens,  and  gives  an  impres- 
sion of  the  gelatine  relief,  being  in  high  relief  where 
the  gelatine  relief  is  so,  and  the  contrary  where  it  is  in 
low  relief. 

It  appears  as  if  a  printing  plate  might  be  easily 
obtained  for  letter-press  from  such  a  gelatine  relief.  Let 
the  gelatine  film  under  a  drawing  be  supposed  to  be 


CHROMO-PHOTOGRAPHY.  233 

exposed.  The  black  lines  then  keep  back  the  light; 
accordingly,  the  gelatine  particles  come  out  in  high  relief 
on  being  wetted  with  water.  The  drawing  is  therefore 
represented  in  high  relief,  and  this  is  exactly  what  the 
printer  requires ;  nothing  further  would  now  be  required 
than  to  recast  the  relief  in  gypsum,  and  recast  the 
gypsum  form  in  metal,  as  happens  daily  in  the  stereo- 
typing of  woodcuts.  But  unfortunately  this  process 
breaks  down,  owing  to  a  trifling  circumstance, — the 
strokes  are  of  unequal  length  in  the  relief  wetted  with 
water.  But  the  letter-press  requires  the  strokes  to  be 
on  a  plane  surface,  otherwise  they  cannot  be  equally 
inked  and  printed. 

On  the  other  hand,  the  casting  can  be  very  well  applied 
as  a  picture  in  relief,  if  suitable  retouches  are  given  to 
it.  Beliefs  of  this  kind  with  portraits  were  sold  some 
years  ago  as  sealing-wax,  but  the  execution  was  very 
imperfect,  therefore  these  reliefs  soon  lost  favour. 
Metallic  forms  are  prepared  according  to  this  process 
in  the  Imperial  Bussian  expedition  for  the  production 
of  State  papers,  and  these  forms  can  produce,  when 
printed  on  paper,  water-marks  which  are  used  in  the 
production  of  bank-notes. 

But  reliefs  can  be  obtained  in  another  way,  from  an 
exposed  film  of  gelatine,  mainly  by  hot  water.  As  we 
have  seen  above,  this  dissolves  the  parts  which,  not  having 
been  affected  by  light,  have  remained  'soluble,  and  it 
leaves  the  parts  affected  by  light,  and  therefore  in- 
soluble. These  parts  that  have  remained  insoluble 
stand  out  as  prominences. 

Another  precaution  is  necessary.  Suppose  that  N 
(Fig.  93  a)  is  a  negative,  that  c  c  are  its  transparent 


234 


THE    CHEMISTEY   OF  LIGHT. 


parts,  and  b  the  semi-transparent,  what  are  called  half- 
tones. If  a  film  of  gelatine  and  salts  of  chromium  g 
(Fig.  93  b)  is  exposed  under  them,  the  light  penetrates 
in  various  degrees,  according  to  its  strength — most  in 
the  transparent  places,  less  in  the  half-transparent,  and 
not  at  all  in  the  opaque  parts. 

Accordingly,  insoluble  films  of  different  thickness  will 


Fig.  93. 

be  formed,  as  represented  Fig.  93.  The  shaded  parts 
in  the  figure  denote  the  portions  that  have  become 
insoluble. 

If  now  the  film  of  gelatine  (Fig.  93  6)  is  plunged  in  hot 
water,  all  the  parts  left  white  in  the  figure  become  dis- 
solved ;  but  at  the  same  time  the  half-tones  not  adhering 
to  the  substratum  P — for  instance,  paper — become 


CnROMO -PHOTOGRAPHY.  235 

detached  and  are  torn  off.  Therefore  a  relief  of  the  form 
Fig.  93  d  remains  behind;  the  half-tones  are  wanting 
(y  y).  In  order  to  avoid  this  interruption  the  new  sub- 
stratum must  be  given  to  the  exposed  surface,  which 
retains  the  half-tones.  For  this  purpose  a  piece  of 
albuminized  paper  is  pressed  on  the  exposed  surface, 
after  being  very  firmly  fixed  to  the  upper  surface  of  the 
gelatine  film.  If  the  sheet  (Fig.  93  b)  is  plunged  in  hot 
water,  the  membrane  P  becomes  detached  from  g,  the 
little  portions  of  gelatine  remain  suspended  to  the 
albuminized  paper,  the  white  places  in  Fig.  93  b 
become  dissolved,  and  all  the  half-tones  y  y  adhere 
firmly  to  the  new  layer,  as  in  Fig.  93  et  and  form  a  relief. 
To  economize  these,  the  gelatine  film  can  be  produced 
on  a  transparent  collodion  film,  and  exposed  to  the  light 
on  the  reverse  side  under  the  negative ;  the  result  is 
then  the  same  as  in  Fig.  93.  This  process  is  named  the 
transferring  process.  If  the  relief  produced  by  cold 
water  sprinkling,  described  p.  232  (Fig.  93  c),  is  com- 
pared with  that  produced  with  hot  water  (Fig.  93  e),  the 
difference  is  at  once  apparent:  in  the  former  case  the 
parts  not  exposed  stand  out  in  relief,  in  the  latter  case 
those  exposed  to  light. 

SECTION  IY.— PRINTING  IN  RELIEF, 

Production  of  Photographic  Half-tones — Production  of  a  Printing  Plate 
in  Belief  from  a  Gelatine  Relief — Woodbury's  Printing  Process — Its 
Importance— Printing  in  Relief  on  Glass,  and  Magic-lantern 
Pictures. 

The  production   of  reliefs  with   cold   and   also  with 
warm  water,  which  we  have  described  in  the  previous 


236  THE    CHEMISTRY   OF   LIGHT. 

chapter,  did  not  lead  the  way  to  a  kind  of  photo-sculpture, 
but  to  a  peculiar  process  of  printing,  which  has  been 
called  printing  in  relief,  from  its  mode  in  operation,  and 
was  invented  by  Woodbury  in  England,  in  1865. 

The  heliographic  methods  of  printing  appear  to  be 
very  simple,  but  they  are  unable  'to  reproduce  images  of 
all  objects  on  plates  that  can  be  used  for  printing.  An 
outline  drawing  or  letter-press  can  be  tolerably  well 
reproduced  by  this  method,  on  an  enlarged  as  well  as 
a  diminished  scale,  and  it  is  this  that  gives  value  to  the 
process.  But  it  is  much  more  difficult  to  reproduce  in 
this  manner,  from  nature,  pictures  with  half-tones ;  for 
example,  stippled  drawings  and  photographs.  The 
tender  half-tones  become  rough  and  hard,  rendering  the 
picture  very  ugly.  According  to  Osborne,  the  cause  of  this 
is  found  chiefly  in  the  nature  of  half-tones  in  copper- 
plate printing.  The  half-tone  in  copper-plate  printing 
is  produced  by  the  fact  that  black  strokes  of  various 
thickness  are  placed  beside  each  other,  as  can  be 
detected  in  engraved  copper-plates  and  in  ordinary 
woodcuts;  or  it  is  effected  by  roughing  the  plate,  thus 
forming  a  series  of  points,  which  appear  more  or  less 
grey  or  black,  according  as  they  are  nearer  to  or  farther 
from  each  other,  and  thus  they  form  the  half-tones. 
The  half-tone  of  stippled  drawings  and  photographs  is 
quite  different.  It  does  not  form  strokes  or  points,  but 
a  homogeneous  light  or  dark  colour. 

Accordingly,  it  was  first  necessary  in  a  manner  to 
break  up  the  photographic  half-tone  into  a  series  of 
strokes  or  points,  in  order  for  it  to  become  a  copper- 
plate half-tone,  and  this  constitutes  the  difficulty. 

Woodbury  conceived  the  idea  to  produce,  by  a  new 


CHROMO-PHOTOGRAPHY.  237 

printing    process,     homogeneous     half-tones,    perfectly 
similar  to  those  of  photographs  or  stippled  drawings. 

He  represented  a  relief,  by  exposing  a  film  of  chromo- 
gelatine  resting  on  collodion  under  a  negative,  and  on 
the  reverse  side,  and  by  treating  it  with  hot  water.  (See 
last  chapter.)  This  relief  shows  the  inky  parts  of  the 
original  in  high  relief,  and  the  light  parts  in  low  relief. 
For  the  negative  is  transparent  where  the  original  is 
black;  hence  the  light  passes  unimpeded  through  those 
places.  The  half-tones  imprint  themselves  by  varying 
between  high  and  low  relief.  They  are,  as  it  were,  the 
declivities  of  the  heights.  (Compare  Fig.  93  e). 

If  this  gelatine  relief  is  suffered  to  dry,  it  becomes 
wonderfully  hard  and  firm.  It  can  then  be  placed  with 
a  plate  of  lead  under  a  strong  press,  and  an  impression 
of  the  relief  can  be  thus  obtained  in  lead.  The 
prominent  parts  of  the  gelatine  relief  appear,  of  course, 
depressed  in  the  lead,  and  the  depressions  prominent,  as 
represented  Fig.  93  d. 

Woodbury  uses  this  lead  relief  as  a  printing  plate. 
But  he  does  not  print  it  off  with  oily  printer's  ink,  for  it  is 
too  opaque,  but  with  a  semi-transparent  gelatine  ink. 
This  is  poured  warm  on  the  plate  in  a  horizontal 
position,  it  penetrates  into  the  depressions,  and  now,  if  a 
piece  of  paper  be  placed  upon  it  and  pressed  gently 
down,  the  gelatine  consolidates  quickly,  and  an  impres- 
sion resembling  relief  is  obtained  on  the 'paper.  As  the 
ink  is  transparent,  it  appears  in  its  thin  coats  much  less 
black  than  in  the  thick  coats,  and  in  places  where  its 
thickness  continually  diminishes  occurs  a  transition 
from  black  to  white — a  perfectly  homogeneous  white 
tone.  As  soon  as  the  coating  dries,  the  relief  contracts 


238  THE    CHEMISTRY   OF   LIGHT. 

considerably;  but  the  semi-transparency  remains,  and 
thus  it  is  possible  to  reproduce  the  most  beautiful  half- 
tones of  photography  by  printing.  This  relief-printing 
of  Woodbury  has  already  attained  a  high  importance. 
It  can  work  with  any  colour,  and  it  facilitates  the 
multiplication  of  photographic  negatives  by  a  single 
printing  form,  without  the  help  of  light.  It  is  therefore 
of  importance  where  a  great  number  of  pictures  are 
required;  for  example,  in  reproduction  of  oil-paintings 
and  drawings  by  the  Eelief  Printing  Company  in 
London,  Carbutt  in  Philadelphia;  Goupil  and  Co.  in  Paris, 
and  Bruckmann  at  Munich.  Photographers  do  not  use 
it  much  in  portrait  taking,  because  the  production  of 
a  faultless  gelatine  relief  and  its  impression  on  lead 
require  a  long  process  and  an  expensive  apparatus, 
which  would  not  pay  in  the  limited  sphere  of  portrait 
photography. 

The  frontispiece  of  the  present  work,  representing  the 
moon,  from  a  photograph  of  Eutherford  (mentioned 
p.  193),  is  an  impression  in  relief,  done  by  the  Eelief 
Printing  Company  in  London. 

It  is  a  special  advantage  of  the  relief-printing  process 
that  it  admits  of  printing  on  glass.  Wonderful  trans- 
parencies are  thus  obtained,  very  effective  as  window 
blinds.  Goupil  has  prepared  copies  of  oil-paintings  in 
this  style  of  relief,  and  they  are  frequently  to  be  seen 
in  the  windows  of  our  dealers.  The  transparent  stereo- 
scopic pictures  on  glass,  produced  by  this  process,  are 
equally  charming — in  sharpness  and  softness  they  almost 
exceed  the  ordinary  silver  copies.  Eecently  a  number 
of  beautiful  magic  lantern  pictures,  produced  by  the 
Woodbury  press,  have  been  offered  for  sale,  and  will  be 


CHROMO-PHOTOGKAPHY.  239 

eventually  used  as  an  important  means  of  instruction  in 
schools.  The  author  has  a  collection  of  American  land- 
scapes of  this  kind,  which,  when  enlarged  in  the  magic 
lantern,  are  more  instructive  than  the  fullest  geographical 
treatise. 

Pictures  of  this  kind  can  be  sold  much  cheaper  than 
the  ordinary  transparent  photographs  for  stereoscopes. 
(See  the  chapter  on  landscape  photography,  p.  163.) 


SECTION  V. — PIGMENT  PRINTING,  OR  THE  PRODUCTION  OP 
CHARCOAL  PICTURES. 

Poitevin' s  Process — Production  of  Pictures  in  any  kind  of  Pigment — 
Its  Difficulty — Inverted  Impressions — Transferring  Process — Com- 
parison of  the  Pigment  Press  with  the  Silver  Press — Braun's  Fac- 
simile from  MSS. — Transference  of  an  Impression  by  Light 
through  Pressure. 

WE  have  seen  above  that  gelatine  mixed  with  chromate 
of  potash  is  insoluble  in  the  light.  This  fact. was  made 
by  its  discoverer,  Talbot,  the  basis  of  heliography — 
that  is,  of  photographic  steel  engraving.  Poitevin,  a 
Frenchman  who  has  done  much  to  promote  photo- 
graphy, founded  on  the  same  method  another  process : 
he  produced  the  pictures  in  different  pigments  (colours). 
He  first  used  charcoal  as  a  pigment,  and  he  then 
obtained  charcoal  pictures. 

The  process  is  simple  :  Poitevin  took  gelatine  coloured 
with  printing  ink,  placed  paper  over  it,  and  exposed  this 
under  a  negative ;  then  he  washed  the  film  of  gelatine  in 
hot  water,  by  which  means  the  soluble  constituents 
perished,  but  the  insoluble  remained  behind  and  retained 
the  ink  mixed  with  it ;  a  charcoal  picture  was  produced 


240  THE    CHEMISTRY    OF    LIGHT. 

in  this  manner.  Though  this  process  appears  very 
simple,  yet  it  has  its  difficulties,  as  was  remarked  above 
in  treating  of  photo-reliefs.  The  operation  of  the  light 
often  does  not  penetrate  to  the  layer  of  gelatine  film. 
The  half-tones  that  have  become  insoluble  do  not  there- 
fore adhere  firmly,  and  are  detached  in  washing,  as  in 
Fig.  93  e,  p.  234 ;  therefore  the  pictures  must  be  trans- 
ferred before  they  are  placed  in  hot  water.  This  takes 
place  as  described  at  the  end  of  the  chapter  on  photo- 
reliefs.  An  albumenized  sheet  is  pressed  in  the  dark  on 
the  coloured  film  of  the  gelatine,  and  then  the  whole  is 
plunged  into  hot  water ;  on  this  the  half-tones  adhere  to 
the  paper  pressed  upon  them,  and  the  image  appears 
uninjured  on  it,  as  in  Fig.  93  e. 

The  position  is,  no  doubt,  reversed ;  that  is,  what  was 
originally  to  the  right  in  the  lower  image  comes  now  to 
the  left.  It  can  easily  be  proved  that  this  can  be  so. 
Let  a  word  be  written  with  thick  ink,  in  large  letters,  on 
paper,  and  Jet  a  piece  of  blotting-paper  be  placed  on  the 
fresh  writing,  after  which  let  the  latter  be  removed ;  in 
this  case  the  writing  in  ink  has  become  printed  off,  but 
reversed.  In  the  letter  copying-press  the  same  thing 
takes  place,  therefore  the  letters  are  printed  on  very  thin 
paper,  that  they  may  be  read  on  the  reversed  side, 
because  viewed  from  that  side  they  appear  in  their  first 
position.  Pigment  impressions  cannot  be  printed  on 
such  thin  paper ;  therefore,  if  the  reversed  position  is 
inconvenient,  another  transferring  process  must  be 
adopted,  by  bringing  the  image  from  the  first  layer  to 
a  second.  This  end  has  been  latterly  attained  in  the 
following  manner : — 

The  exposed  gelatine  surfaces  are  placed  damp  on  a 


CHROMO-PHOTOGBAPHY.  241 

smooth  zinc  table,  and  then  suffered  to  dry.  They  then 
adhere  very  firmly  to  it.  The  copy  thus  glued  to  zinc  is 
plunged  into  warm  water,  it  is  then  developed,  the  paper 
becomes  detached,  and  the  image  lies  on  the  zinc 
table.  A  piece  of  white  glue  paper  is  now  taken,  stuck 
upon  the  zinc  table,  and  allowed  to  dry.  The  image 
then  adheres  firmly  to  the  glued  paper,  and  being  care- 
fully loosened  it  parts  from  the  zinc  table  and  remains 
lying  upon  the  paper.  Then  it  appears  in  its  proper 
position  on  the  paper.  In  this  more  recent  form  the 
process  is  put  in  practice,  especially  at  Woolwich 
Arsenal,  in  England. 

The  pigment  impressions  thus  obtained  resemble,  ex- 
ternally, the  "Woodbury  images,  but  they  surpass  them 
in  their  fineness  and  the  facility  of  their  production. 
But  this  process  has  not  yet  supplanted  the  silver  press, 
for  the  expense  of  the  material,  owing  to  the  twofold  use 
of  paper,  equals  that  of  silver  photography,  and  the 
labour,  being  somewhat  more  complicated,  is  therefore 
dearer.  The  pigment  impressions  have  a  great  advan- 
tage through  the  optional  choice  of  colour;  genuine 
Indian  ink  may  be  used  for  them,  and  then  perfectly 
durable  pictures  are  obtained  that  do  not  turn  yellow 
or  black. 

In  the  same  way  English  red,  sepia  blue,  and  so  on, 
can  be  mixed  with  the  gelatine,  and  thus  pictures  can 
be  produced  in  those  colours.  This  circumstance  is  im- 
portant when  it  is  wished  to  reproduce  manuscripts,  when 
these  have  been  written  in  coloured  characters.  Quantities 
of  such  manuscripts  and  sketches  of  the  old  masters  are 
in  various  museums.  Braun  of  Dornach,  in  Alsace,  the 
same  photographer  who  made  himself  conspicuous  for  his 


242  THE    CHEMISTRY   OF   LIGHT. 

Swiss  views,  has  undertaken  to  reproduce  these  manu- 
scripts in  their  original  colour  by  the  pigment  press,  pre- 
paring first  a  silver  negative  in  the  usual  way,  and  copying 
this  on  coloured  gelatine  films.  In  this  way  he  has  made 
accessible  to  all  artists  and  amateurs,  in  faithful  fac- 
similes, for  a  small  sum,  many  drawings  that  only 
existed  as  a  single  copy. 

Latterly,  a  very  interesting  observation  has  been  made 
by  Abney,  in  England,  in  relation  to  the  pigment  press. 
He  remarked  that  if  an  exposed  film  of  gelatine  remained 
a  long  time  in  the  dark  the  insolubility  increases. 
Accordingly,  a  film  of  this  kind  which,  freshly  developed, 
would  only  give  a  faint  image,  after  lying  some  hours 
gives  a  strongly-defined  image.  This  fact  allows  the 
time  of  exposure  for  pigment  pictures  to  be  considerably 
reduced;  that  is,  several  pictures  to  be  made  at  the  same 
time. 

Still  more  interesting  is  an  observation  of  Marion,  at 
Paris.  He  exposed  a  piece  of  common  paper  that  had 
been  made  sensitive  by  plunging  into  a  solution  of 
chromium,  then  he  pressed  it  in  the  dark  with  a  damp 
sheet  of  pigment  saturated  with  chromate  of  potash; 
the  film  of  pigment  was  thereby  insoluble  in  all  places 
where  it  came  in  contact  with  the  exposed  parts  of  the 
chromic  paper — it  adhered  closely  to  these  parts  of  the 
chromic  paper,  and  on  developing  with  hot  water  he 
obtained  a  pigment  picture  on  the  chromic  paper. 

The  future  will  show  how  far  this  process  is  practically 
useful,  for  there  are  always  difficulties  in  practice,  which 
require  long  experience  to  overcome  them. 


CHROMO-PHOTOGRAPHY.  243 

SECTION  VI. — LIGHT-PRINTING. 

The  Susceptibility  of  exposed  Chromic- Gelatine  to  the  Influence  of 
thick  Printer's  Ink — Services  of  Poitevin  and  Tessie  de  Mothay — 
Albert-type,  or  Light-press — Its  Mode  of  Operation — Its  Use  and 
Comparison  with  Eelief-Printing. 

We  have  seen  that  a  film  of  gelatine  and  chromium  is 
insoluble  in  light,  and  loses  its  tendency  to  liquify.  At 
the  same  time,  the  exposed  places  assume  a  peculiar 
property — they  are  susceptible  to  the  influence  of  thick 
printer's  ink.  If  an  exposed  sheet  of  gelatine  and 
chromium  is  rubbed  with  a  moist  sponge,  it  only  imbibes 
water  in  the  places  untouched  by  the  light,  but  if  it  is 
then  overlaid  by  thick  printer's  ink,  it  is  singular  that 
these  places  only  adhere  to  the  parts  affected  by  light. 
This  fact  was  discovered  by  Poitevin,  the  meritorious 
discoverer  in  photographic  chemistry.  If  a  piece  of 
paper  be  placed  on  such  a  film  of  gelatine,  blackened  by 
ink,  and  it  is  pressed,  the  ink  adheres  to  the  paper  and 
an  impression  of  the  image  is  obtained,  under  whose 
negative  the  film  of  gelatine  had  been  exposed. 

In  this  manner  what  is  called  a  light-print  is  obtained. 
This  peculiar  mode  of  printing  offered  at  first  very  im- 
perfect results.  The  process  was  rendered  unproductive 
from  the  fragile  nature  of  the  gelatine  film,  the  difficulty 
of  finding  the  right  time  for  exposure,  the  proper  con- 
sistency of  the  thick  printer's  ink,  and  other  obstacles. 
After  a  hundred  impressions,  the  filnf  of  gelatine  was 
generally  so  injured  that  it  was  unserviceable.  Tessie 
de  Mothay,  at  Metz,  acquired  some  skill  in  practising 
the  process,  but  Albert,  of  Munich,  was  the  first  to 
develop  it,  so  that  it  has  become  of  practical  importance. 

The  experimenters  before  Albert  had  conveyed  the 


214  THE    CHEMISTRY  OF   LIGHT. 

gelatine  film  to  metal,  but  it  only  adhered  to  this  im- 
perfectly. Albert  poured  the  gelatine  solution,  decom- 
posed with  chromate  of  potash  in  the  dark,  on  glass, 
and  exposed  its  reverse  side  after  drying  for  a  moment. 
In  this  way  the  light  produced  a  superficial  effect,  the 
part  of  the  gelatine  adhering  immediately  to  the  glass 
became  insoluble,  and  was  fixed  wonderfully  firmly  to 
the  glass.  The  film  of  gelatine  was  then  covered  on 
its  upper  surface  with  a  negative,  and  exposed  to  the 
light.  A  faint  greenish  picture  is  thus  produced.  The 
exposed  film  is  then  washed  in  water  until  all  salts  of 
chromium  is  removed,  and  then  it  is  suffered  to  dry. 

In  order  to  print,  a  sponge  is  moistened  with  water 
containing  glycerine,  and  the  film  is  carefully  rubbed 
with  it;  the  water  penetrates  in  all  places  where  the 
light  has  not  operated.  A  leather  cylinder  is  now  taken 
and  inkecl, — that  is,  some  thick  printer's  ink  is  spread 
over  a  piece  of  marble  by  rolling  the  leather  cylinder 
over  it  until  the  surface  is  coated, — then  the  gelatine  film 
is  subjected  to  a  light  pressure  from  the  leather  roller, 
which  is  passed  over  it,  and  this  process  is  frequently 
repeated.  All  places  which  have  been  affected  by  the  light 
receive  ink  from  the  roller,  but  not  so  the  others,  and 
finally  a  strongly-defined  picture  appears  on  the  origin- 
ally almost  colourless  surface.  As  soon  as  this  has  been 
sufficiently  inked,  a  piece  of  paper  is  pressed  upon  it, 
and  it  is  allowed  to  pass  over  a  layer  of  gum,  between 
rollers  also  coated  with  it.  In  this  manner  the  ink  of 
the  picture  passes  over  to  the  paper,  and  produces  thus 
an  impression  with  all  half-tones.  The  inking  and 
printing  can  be  repeated  at  option,  and  thus  thousands 
of  copies  may  be  prepared  if  the  plate  is  very  firm. 


CHROMO-PHOTOGRAPHY.  245 

These  Albert-types,  or  light-impressions,  as  they  have 
been  latterly  called,  approach,  but  do  not  equal,  the 
silver  copies  in  beauty.  The  process  is  well  adapted  to 
reproduce  pencil  and  chalk  drawings,  which  they  re- 
produce with  the  utmost  fidelity.  Herr  Albert  has 
reproduced  and  published  "  Schwind's  Fairy  Tale  of  the 
Seven  Bavens,"  and  several  cartoons  of  Kaulbach,  by 
light-impressions.  In  like  manner,  the  views  of  the 
photographic  detachment  of  the  Prussian  general-staff 
in  the  French  war  have  been  reproduced  by  Obernetter 
in  light-impressions.  The  views  of  the  Vienna  exhibition, 
sold  at  the  building,  and  by  many  supposed  to  be  ordinary 
photographs,  are  light-prints  of  Obernetter  of  Munich, 
wTho  with  Albert  has  done  the  most  in  this  matter.  In 
the  annexed  double  picture  of  Fraulein  Artot  we  give 
our  readers  a  specimen  of  a  light-print  of  Obernetter. 

The  brilliancy  of  these  pictures  is  occasioned  by 
giving  them  a  coat  of  varnish.  If  the  results  of  the 
Woodbury  printing  are  compared  with  those  of  light- 
printing,  it  appears  that  the  relief-printing  gives  the 
shades  and  dark  parts  better,  but  that  the  white  parts 
often  appear  discoloured.  On  the  other  hand,  the  relief- 
prints  are  much  more  like  photographs  than  the  light- 
prints,  for  the  latter  have  a  lithographic  tone.  It  is 
only  by  coating  with  varnish  that  they  are  made  more 
like  photographs.  But  both  methods  are  rather  inferior 
to  ordinary  silver  photography,  which  'has  never  been 
surpassed  in  the  uniformity  of  half-tones,  in  the  beauty 
of  lights,  the  uniformity  of  half-tones,  and  the  depth 
of  shadows,  and  which  has  one  special  advantage  over 
light-printing  and  Woodbury  printing,  and  that  is  the 
ease  of  production.  To  prepare  relief  and  light  impres- 


246  THE    CHEMISTRY   OF   LIGHT. 

sions,  the  first  thing  needed  is  a  printing  plate,  needing 
more  complicated  preparation  than  the  photographic 
positive  process,  and  also  requiring  a  clever  printer. 
But  silver  printing  gives  good  results  with  simple 
means,  and  even  in  inexperienced  hands.  It  will  there- 
fore always  be  preferred  in  portrait  taking,  where  the 
object  is  often  only  to  throw  off  a  dozen  pictures :  but 
light-printing  and  relief-printing  are  of  great  importance 
when  the  object  is  to  produce  large  numbers  of  pictures 
in  a  short  time. 

SECTION  VII. — ANILINE  PRINTING. 

Aniline  Colours  and  their  Origin — Effect  of  Chromic  Acid  on  Aniline — 
Its  Use  in  Photography — Willis's  Printing  Process — Its  Application. 

Every  one  knows  in  modern  times  the  brilliant  aniline 
colours — Hofrnann's  violet,  magenta  red,  aniline  green, 
and  others.  We  are  indebted  for  these  wonderful  pig- 
ments, surpassing  all  before  them  in  brilliancy,  depth, 
and  reflecting  power,  especially  to  the  noted  chemist, 
Hofmann.  The  colours  are  due  to  the  effect  of  different 
substances  giving  out  oxygen  on  aniline. 

Aniline  is  a  substance  which  resembles  ammonia  in 
its  chemical  relations,  only  it  has  a  different  odour  and 
a  different  composition.  The  substance  is  obtained  as  a 
brown  mass  from  coal  tar  on  distillation. 

If  this  brown  fluid  is  treated  with  chloride  or  nitric  acid, 
or  manganese  and  sulphuric  acid,  or  arsenious  acid, 
different  shades  are  produced.  One  specially  interesting 
us  is  the  colour  which  is  created  when  aniline  is  heated 
with  chromate  of  potash  and  sulphuric  acid ;  the  result 
is  a  peculiar  violet  substance — aniline  violet.  Chromate 
of  potash  plays  a  part  in  our  photo -chemical  processes, 


CHKOMO  PHOTOGRAPHY.  247 

and  on  this  is  based  the  aniline -printing  invented  by 
Willis. 

Willis  plunges  a  piece  of  paper,  in  the  dark  chamber, 
in  a  solution  of  chromate  of  potash  and  sulphuric  acid ; 
he  exposes  the  paper  under  a  positive  picture,  e.g.  a 
drawing  or  copper  engraving.  The  light  shines  through 
the  white  paper,  and  in  these  places  the  chromic  acid  is 
reduced  to  chromic  oxide,  which  does  not  affect  aniline 
colours ;  on  the  other  hand,  the  chromic  acid  remains 
unchanged  under  the  black  strokes,  which  keep  back  the 
light.  After  the  exposure  a  very  pale  picture  is  seen 
of  unchanged  yellow  chromic  acid. 

If  this  faint  picture  is  exposed  to  fumes  of  aniline,  at 
the  places  where  the  yellow  strokes  exist  aniline  brown 
is  formed,  and  in  this  manner  the  original  pale  yellow 
becomes  very  defined.  This  fumigation  with  aniline  is 
effected  by  placing  the  copies  in  a  box  and  covering 
them  with  a  cover,  having  a  layer  of  blotting-paper  on 
its  lower  surface  moistened  with  a  solution  of  aniline 
with  benzine.  This  process  reproduces  a  positive  picture 
of  a  positive,  and  is  therefore  very  valuable  in  producing 
faithful  copies  of  drawings.  I  admit  that  these  copies 
are  reversed  in  position,  as  is  the  case  with  mirrors. 
This  circumstance  limits  their  use  in  many  cases.  We 
have  already  explained  the  reason  of  this  reversing  of 
copies  (p.  240).  Copies  can,  however,  be  obtained  in 
their  proper  position  if  the  original  drawing  is  very  thin. 
In  that  case  the  reversed  side  of  the  drawing  is  placed 
against  chromic  acid  paper,  and  the  light  is  suffered  to 
shine  through  on  it  from  the  upper  surface. 

It  is  another  disadvantage  of  this  process  that  the 
chromic  acid  paper  must  be  always  freshly  prepared, 


248  THE    CHEMISTRY    OF    LIGHT. 

as  it  quickly  spoils,  and  the  duration  of  the  copying 
must  be  very  carefully  estimated.  If  the  time  is  too 
short,  unchanged  chromic  acid  remains  everywhere  on 
the  paper,  and  then  it  all  blackens  in  fumes  of  aniline. 
If,  again,  the  time  of  copying  takes  too  long,  the  light 
works  gradually  through  the  black  strokes  of  the  drawing, 
reduces  the  chromic  acid,  and  the  paper  then  remains 
entirely  white  in  the  aniline  fumes,  as  no  more  chromic 
acid  is  present  to  form  aniline  colours.  These  circum- 
stances limit  the  value  of  the  method,  and  cause  the 
licht-paus  process  (p.  25)  to  be  preferred  to  it.  In 
England  the  aniline  printing  is  practised  by  the  inventor, 
who  prepares  copies  to  order. 

SECTION  VIII. — PHOTO-LITHOGRAPHY. 

Nature  of  Lithography — The  Lithographic  Colour-Press — Zincography 
— Poitevin's  Discovery — Photo-Lithography — Its  Application  in 
multiplying  Maps  quickly — Its  Importance  in  War — Difficulties — 
The  Anastatic  Method — Photo- Lithography  with  Asphalt. 

By  lithography  is  understood  printing  off  from  a  drawn 
or  painted  stone.  Near  the  little  Bavarian  town  of 
Solenhofen,  there  is  a  clayey,  rather  porous  limestone, 
easily  polished  and  wrorked.  Such  limestone  is  used  for 
lithography.  But  the  lithographic  press  differs  essen- 
tially from  copper  printing  and  book  press,  because  the 
drawing  on  stone  is  neither  raised  nor  incised.  The 
lithographic  stone  forms,  in  fact,  with  its  image  intended 
for  printing,  a  smooth  surface;  and  this  process  is 
peculiar,  differing  from  all  other  modes  of  printing.  If  a 
drawing  is  made  on  a  lithographic  stone  with  chalk,  or 
ink  consisting  of  colour  and  a  fatty  substance,  e.g.  oil, 
and  if  the  stone  is  moistened  with  water,  it  only  penetrates 


CHROMO -PHOTOGRAPHY.  249 

the  porous  stone  where  there  is  no  oily  colour,  for  oil 
repels  water.  Oily  printer's  ink,  similar  to  printer's  ink, 
only  considerably  more  delicate,  is  rubbed  on  the  stone 
with  a  leather  roller ;  it  only  adheres  where  there  is  the 
oily  ink — that  is,  to  the  drawing. 

After  the  stone  has  been  inked  as  above,  if  a  piece  of 
paper  is  pressed  upon  it  the  ink  passes  over  to  it,  and  a 
lithographic  impression  is  obtained.  The  stone  can  be 
evidently  inked  and  used  at  discretion,  and  thus 
thousands  of  copies  can  be  produced.  This  style  of. 
printing  has  many  advantages  over  copper  engraving. 
The  working  of  a  copper  plate  is  a  difficult  matter,  often 
requiring  a  labour  of  years  to  engrave,  whereas  the 
working  on  stone  is  much  easier — almost  as  easy  as 
drawing  on  paper.  In  like  manner,  printing  from  a 
stone  plate  has  fewer  difficulties  than  that  from  a  copper 
plate.  The  stone  easily  admits  of  corrections  in  drawing, 
and  after  being  used  once,  the  same  stone  will  serve 
again,  and  often  for  many  years.  These  circumstances 
have  brought  lithography  into  general  use :  technical 
drawings,  wine  cards,  pictures  of  saints,  notes,  visiting 
cards,  lists  of  prices,  calendars,  illustrations  of  books, 
atlases,  pictures  of  natural  history,  and  a  thousand  other 
things  are  produced  by  lithography ;  and  latterly  a  special 
field,  called  chromo-lithography,  has  obtained  a  large 
development.  It  is  the  most  important  of  existing  pro- 
cesses to  produce  painted  pictures  in  a  mechanical  way. 
Chromo-lithography  is  rather  more  complicated  than 
common  lithography.  If  it  is  wished  to  make  a  chromo- 
lithograph of  a  painted  picture,  not  only  one  stone,  but  a 
separate  stone  for  almost  every  colour  must  be  prepared. 
For  example,  if  you  have  an  object  in  which  the  blue,  red, 


250  THE    CHEMISTRY   OF   LIGHT. 

and  yellow  tones  appear,  you  first  draw  on  a  stone  that 
is  to  contain  the  blue  places,  and  then  colour  it  blue ;  a 
second  and  third  stone  are  required  for  the  yellow  and 
red  places.  All  three  stones  are  printed  off,  in  a  position 
corresponding  to  the  colours,  on  the  same  piece  of  paper. 
They  then  produce  a  painted  impression  which,  if  an 
oleograph  is  required,  must  be  coated  with  a  brilliant 
varnish.  Though  chromo-lithography  offers  great  advan- 
tages for  maps,  ornaments,  etc.,  and  affords  many 
^excellent  artistic  specimens — e.g.  the  chromo -lithographs 
of  Hildebrandt's  water-colour  views — we  must  express  an 
adverse  opinion  against  oleography,  which,  with  a  few 
honourable  exceptions — Prang  at  Boston,  Korn  at  Berlin, 
and  Seitz  at  Hamburg — produces  pictures  of  very  small 
artistic  value,  and  has  done  much  to  injure  the  public 
taste. 

A  perception  of  colour  and  a  feeling  for  art  are 
necessary  for  chromo-lithography,  and  the  printers  do 
not  possess  them. 

Closely  related  to  lithography  is  zincography,  which 
we  shall  glance  at  here  before  passing  to  photo-litho- 
graphy. 

It  is  remarkable  that  zinc  has  properties  similar  to 
those  of  the  lithographic  stone.  It  easily  receives 
drawings  with  soft  chalk,  and  after  being  moistened 
with  gum  water,  it  can  be  as  easily  rolled  as  a  stone 
with  moist  colours,  the  colour  adhering  then  to  the  places 
drawn.  The  printing,  therefore,  presents  results  similar 
to  those  of  lithography;  but  the  preparation  for  zinc 
printing  has  more  difficulties  than  lithography,  so  that 
the  use  of  zinc  for  this  purpose  is  limited. 

We  have  only  given  a  brief  survey  of  the  principles  of 


CHROMO-PHOTOGRAPHY.  251 

lithography  and  zincography,  as  far  as  necessary  to 
understand  what  follows.  Both  processes  resemble  light- 
printing  in  many  respects;  the  light-press  surface  has 
also  the  peculiarity  of  receiving  thick  ink  in  some 
places  and  repelling  it  in  others, — but  light -printing  is  of 
recent  date,  while  lithography  has  existed  seventy  years. 
When  photography  was  invented,  it  deprived  lithography 
of  an  important  branch,  that  of  portraits.  Even  in  1850 
numerous  lithographic  portraits  were  made  of  indi- 
viduals. But  since  the  introduction  of  cartes  de  visite 
at  that  date,  portrait  lithography  has  greatly  fallen  off, 
and  is  only  used  for  cheap  likenesses  of  distinguished 
persons.  The  lithographs  from  oil-paintings  have  also 
suffered  through  photography,  which  thus  entered  into 
competition  with  lithography.  It  was  Poitevin  who 
allied  the  two  by  inventing  photo-lithography.  It  was 
Poitevin's  aim  to  economize  the  labour  of  the  litho- 
graphic draughtsman,  and  to  replace  it  by  the  chemical 
operation  of  light.  He  coated  lithographic  stones  with 
chromate  of  potassium  and  gelatine,  and  exposed  them 
under  a  photographic  negative.  The  chromo -lithograph 
thus  obtained  was  then  washed  and  rolled.  All  parts 
affected  by  light  took  the  colour,  and  gave  an  impres- 
sion in  the  press.  The  first  attempts  of  the  kind  were 
very  imperfect ;  the  pictures  were  especially  wanting  in 
half-tones,  which  were  lost  in  washing,  as  they  are  in 
the  pigment  press  (see  p.  239) ;  therefore  Asser  and 
Osborne  attempted  another  manner,  what  is  called 
re-impression.  They  copied  their  negatives  on  chromic 
paper,  partly  coated  with  gum  gelatine  or  albumen,  and 
then  they  rolled  it.  Chromic  paper  has  the  peculiarity 
of  receiving  thick  printer's  ink  on  the  exposed  places 


252  THE   CHEMISTRY   OF   LIGHT. 

after  exposure.  After  inking,  the  chromic  paper  was 
carefully  washed  and  then  pressed  on  a  lithographic 
stone.  This  sucked  up  the  colour,  and  thus  the  picture 
was  perfectly  transferred  to  the  stone.  The  stone  thus 
prepared  gave  excellent  impressions  in  the  usual  style 
of  lithography.  Though  half-tones  were  thus  produced, 
the  impression  fell  Mr  short  of  photography  in  quality. 
The  lithographic  half-tone  differs  essentially  from  the 
photographic,  which  forms  a  homogeneous  surface,  while 
the  lithographic  appears  as  a  mass  of  black  points  more 
or  less  near  together.  The  granulous  nature  of  the 
stone  does  not  allow  such  delicacy  of  reproduction  as 
photography  ;  therefore,  photo-lithography  is  employed 
only  where  cheap  production  of  many  copies  is  of 
greater  moment  than  delicacy. 

Quite  recently,  maps  of  rivers  and  mountains  have 
been  sold  by  Kellner  &  Co.,  at  Weimar,  which  have 
been  produced  by  photographing  reliefs  in  gypsum,  and 
copying  the  negatives  obtained  by  photo-lithography. 
They  thus  succeeded  in  producing  maps  of  mountains 
without  the  help  of  a  draughtsman,  and  could  sell  them 
at  an  extremely  low  price. 

Such  photo-lithographs  do  not  answer  as  works  of  art. 
In  this  sphere  light-printing,  with  its  excellent  half-tones, 
is  a  formidable  competitor,  though  photo-lithography  is 
very  useful  by  giving  a  great  number  of  impressions 
from  the  same  plate,  while  the  number  of  light-prints 
given  off  by  a  gelatine  plate  is  always  limited  and  rather 
uncertain. 

In  one  branch  photo-lithography  surpasses  all  other 
reproducing  arts ;  that  is  in  giving  copies  of  maps,  which 
have  first  been  drawn  in  outline.  The  production  of 


CHROMO -PHOTOGRAPHY. 

geographical  maps  is  a  field  requiring  muchiime  and 
observation.  The  individual  outlines  of  mountains, 
rivers,  and  countries  must  be  executed  with  the  greatest 
exactitude,  corresponding  with  measurements.  Fre- 
quently, different  draughtsmen  and  engravers  are  em- 
ployed, and,  though  working  conscientiously,  inaccuracies 
are  unavoidable  and  make  correction  necessary.  All 
this  takes  time  and  trouble.  If  the  object  is  now  to 
take  an  enlarged  or  diminished  copy  of  a  map,  the 
same  difficulties  occur,  and  the  diminishing  is  especially 
troublesome.  The  pantograph  is  a  useful  aid  here,  but 
does  not  exclude  inattention  in  the  draughtsman.  In 
this  respect  photography  is  invaluable  as  an  aid  to 
map-making.  With  very  great  ease,  photography  gives 
enlarged  or  diminished  copies  of  an  original ;  in  a  few 
hours  this  is  copied  on  stone,  and  within  a  day  photo- 
lithography can  throw  off  thousands  of  enlarged,  dimin- 
ished, or  original  sized  copies. 

If  it  were  wished  to  make  a  lithographic  stone  draw- 
ing by  handiwork,  several  days  would  be  wanted,  and  it 
would  be  far  less  exact.  No  photographic  printing 
process  is  as  rapid  as  photo-lithography ;  therefore  map- 
making  has  made  great  use  of  it,  especially  when  very 
numerous  copies  were  required.  In  the  French  war,  the 
advancing  troops  needed,  before  all  things,  maps  of  the 
territory  to  be  occupied.  But  there  was  not  a  sufficient 
supply  of  maps  of  France  to  provide  whole  Army  Corps 
with  them.  It  was  not  to  be  thought  of  before  the  war, 
as  no  one  can  tell  where  a  campaign  will  take  place. 
Photo-lithography  was  here  an  auxiliary,  by  giving 
thousands  of  copies  from  one  original  map ;  and  thereby 
it  contributed  to  the  successful  advance  of  the  German 

12 


254  THE    CHEMISTRY   OF   LIGHT. 

army,  which,  with  these  maps  in  its  hand,  showed  itself 
better  acquainted  with  the  enemy's  territory  than  the 
enemy's  troops  themselves.  The  photo-lithographic 
establishment  of  the  brothers  Burchard,  at  Berlin, 
which  has  displayed  activity  on  a  large  scale,  produced 
in  the  war  time  of  1870-71,  500,000  maps.  Plate  V.  is 
a  specimen  of  this  process. 

Besides  these  contributions,  we  must  mention  the 
photo-lithographic  labours  of  Herr  Korn  at  Berlin,  which 
belong  more  to  the  region  of  art.  Particularly  admir- 
able in  this  branch  are  the  photo-lithographic  pages  of 
the  pen-and-ink  drawings  of  Berg  in  the  Japan  expe- 
dition. These  are  so  faithfully  produced  that  original 
and  copy  cannot  be  distinguished, — I  admit  that  the 
character  of  the  originals  was  very  favourable  to  photo- 
lithographic reproduction.  Faint  drawings  place  diffi- 
culties in  the  way  of  photographic  reproduction,  espe- 
cially when  they  have  a  bluish  tint ;  therefore,  pencil 
drawings  are  very  difficult  to  photograph.  But  no 
perfect  photo-lithograph  can  be  produced  from  an  im- 
perfect photographic  negative.  Thus  far,  therefore,  the 
character  of  the  original  has  a  very  material  influence. 
Berg's  pen  drawings  are  executed  in  strong  black  Indian 
ink,  and  therefore  easy  to  reproduce.  In  the  Austrian 
Institution  of  Military  Geography,  the  map  drawings 
which  are  to  be  copied  by  photo-lithography  are  so 
executed  from  the  first  that  they  take  a  favourable 
photographic  effect,  or,  to  use  the  technical  term,  come 
out  well.  The  photographic  reproduction  of  a  drawing 
is  especially  favoured  by  brown  tints,  such  as  umber 
and  cinnabar,  when  mixed  with  Indian  ink.  On  the 
other  hand,  much  depends  on  the  paper  being  without 


A   SECTION      >F    THE    ORDNANCE      SURVEY  MAP 
OP    WALES. 


- 


CHROMO  -PHOTOGRAPHY,  255 

blemish ;  yellow  spots,  scarcely  visible  to  the  eye,  have 
the  same  effect  in  photography  as  black.  We  knew  a 
case  in  Kron's  photographic  printing-office  where  an 
unblemished  drawing  of  a  map  came  out  as  a  photo- 
graph full  of  spots.  The  defect  was  attributed  to  the 
chemicals,  until  it  was  found  that  minute  rust  spots  in 
the  paper,  which  had  got  into  it  during  manufacture, 
were  the  cause  of  the  defect.  In  such  cases  the  evil 
can  only  be  remedied  by  suitable  negative  retouche. 

The  nature  of  photo-zincography  will  now  be  clear  to 
the  reader :  as  the  zinc  plate  is  so  like  the  stone,  the 
treatment  is  the  same.  The  negative  is  either  copied 
direct  on  the  zinc  plate,  coated  with  gelatine  and 
chromium,  or  a  copy  from  the  negative  is  prepared  on 
chromo-glucose  paper;  the  paper  is  inked  and  trans- 
ferred to  the  zinc  plate,  being  pressed  together  with  it. 
After  this  the  zinc  plate  can  give  impressions. 

It  must  be  remarked  in  this  connection,  that  even 
without  photography,  direct  mechanical  copies  can  be 
made  of  maps,  writings,  etc.,  if  the  original  be  executed 
in  oily  or  analogous  colours.  This  takes  place  by  means 
of  the  anastatic  process.  This  process  is  based  on 
moistening  the  original  on  the  reverse  side  of  the  draw- 
ing with  acidulated  gum-water,  and  then  damping  it 
from  above  with  a  fresh  colour ;  this  only  adheres  to  the 
oily  strokes  of  the  drawing  or  printing^  The  original, 
thus  freshly  inked,  is  then  placed  on  a  fresh  stone,  or  a 
freshly  cleaned  zinc  plate,  and  put  under  a  pressure. 
Then  the  drawing  pass-es  over  to  the  stone  or  the  zinc, 
and  can  be  easily  multiplied  by  rolling  and  printing.  It 
is  difficult  to  preserve  the  original,  which  suffers  greatly 
under  pressure.  Still  more  difficult  is  it  to  reproduce  a 


256  THE    CHEMISTRY   OF   LIGHT. 

pure  stroke,  for  these  are  often  extended  widely  by  the 
pressure,  and  if  the  strokes  are  too  thin  they  run 
together,  as  in  mountain  lines  in  maps ;  therefore  the 
process  has  been  more  applied  to  copy  antiquarian 
books,  which  have  been  reproduced  page  by  page  in  this 
way. 

It  is  self-evident  that  only  reproductions  of  the 
original  size  can  be  made  by  the  anastatic  process. 

We  "have  to  mention  another  process  of  photo-litho- 
graphy, based  on  the  application  of  asphalt.  "We  have 
already  described  this  in  our  first  chapter  as  a  sensitive 
substance,  and  also  a  process  called  heliography,  which 
produces,  by  means  of  photography,  copper  plates  and 
steel  plates  for  printing.  Asphalt  serves  also  for  photo- 
lithography. A  lithographic  stone  is  sprinkled  with  a 
solution  of  asphalt  in  ether,  allowed  to  dry  in  the  dark, 
and  exposed  under  a  negative.  The  asphalt  becomes 
insoluble  on  the  exposed  places,  and  is  retained  upon 
treating  the  stone  with  ether  or  benzine.  If  the  stone 
is  then  damped,  the  moisture  only  penetrates  wrhere  no 
asphalt  covers  the  stone.  On  rolling  it  after  this  with 
oily  ink,  this  is  rejected  from  the  damp  places,  and  only 
adheres  to  the  asphalt — that  is,  to  the  picture ;  thus  a 
stone  giving  impressions  is  obtained.  This  method  gives 
good  results  in  the  hands  of  several  practitioners,  and  is 
preferred  by  many  to  the  chromium  process,  though 
asphalt  is  much  less  sensitive  than  chromium. 


CHROMO-PHOTOGEAPHY.  257 


SECTION  IX. — PYRO-PHOTOGRA.PHY  WITH  SALTS  or  CHROMIUM. 

Poitevin's  Process —  Effect  of  Chromate  of  Potash  on  Sticky  Substances 
— Pictures  Developed  by  Dust — Pictures  on  Porcelain — Oidtmann's 
Pyro-photography — Application  to  the  Decoration  of  Glass — Photo- 
graphy and  Painting  on  Glass. 

Photography  has  become  allied  to  almost  all  the 
multiplying  and  descriptive  arts,  though  it  was  at  first 
looked  upon  as  their  rival.  It  is  not  surprising,  there- 
fore, that  it  has  become  a  help  in  porcelain  painting  and 
decoration.  We  have  already  seen  (p.  212)  the  peculiar 
process  of  changing  silver  pictures  into  gold  and  platinum 
pictures,  transferring  them  to  porcelain,  and  burning 
them  in.  That  method  might  be  called  a  moist  process  ; 
the  same  end  can  be  obtained  by  a  dry  method,  and  by 
the  help  of  salts  of  chromium.  This  original  method  has 
also  been  invented  by  Poitevin,  and  subsequently  was 
materially  improved  by  Joubert  in  London,  and  Ober- 
netter  at  Munich.  It  consists  in  this :  that  a  mixture 
of  gum,  honey,  and  chromate  of  potash  is  poured  on 
glass ;  the  film  is  carefully  dried  in  the  dark,  and  then 
exposed  under  a  positive.  The  film  of  gum  is  freshly 
prepared,  sticky,  and  holds  fast  the  scattered  coloured 
powder,  but  when  the  film  is  exposed,  it  loses  its  sticki- 
ness. If  this  exposure  takes  place  under  a  drawing 
with  black  strokes,  the  film  under  them  will  retain  its 
stickiness,  but  lose  it  beneath  the  wHite,  transparent 
parts  of  the  paper. 

Therefore,  if  the  film,  after  exposure,  be  powdered  in 
the  dark  with  any  colour  in  powder,  this  adheres  where 
the  strokes  of  the  drawing  have  protected  the  film,  but 
not  at  other  places,  and  thus  a  picture  in  powdered 


258  THE    CHEMISTRY   OF   LIGHT. 

colour  is  obtained.  If  this  coloured  powder  and  its  under- 
layer  is  fire-proof — as  glass  and  porcelain — the  picture 
obtained  can  be  burnt  into  it,  and  pictures  of  very 
different  shades  can  be  produced,  according  to  the 
choice  of  the  powdered  colour.  Pictures  of  this  kind  can 
be  transferred  from  one  under-layer  to  another.  If  a 
collodion  film  is  poured  upon  the  powdered  picture,  if 
this  is  suffered  to  dry  and  then  the  whole  thrown  into 
water,  when  there  the  collodion  film  with  the  picture 
can  be  easily  taken  off,  stuck  on,  and  burnt  into  other 
surfaces — glasses,  cups,  etc.  Thus  Joubert  in  London 
has  actually  burnt  in  large  pictures  on  glass.  Obernetter 
at  Munich,  and  Leth  at  Vienna,  Leisner  at  Waldenburg, 
and  Stender  at  Lamspringe,  Greiner  at  Apolda,  and  Lafon 
de  Camarsac  at  Paris  have  produced  encaustic  pictures 
on  porcelain  in  the  same  manner. 

This  process  is  not  much  employed  for  portraits.  On 
the  other  hand,  Oidtmann  at  Linnich,*  has  employed  it 
advantageously  in  glass  manufacture.  He  has  copied 
patterns  of  carpets  from  lithographs  directly  on  glass, 
and  burnt  them  in,  thereby  producing  cheap  window 
ornaments,  which  can  be  painted  and  embellished  by 
the  hand.  At  the  Vienna  exhibition,  there  was  over  the 
door  of  the  Emperor  of  Germany's  pavilion  a  rosette 
ten  feet  in  diameter,  produced  by  Dr.  Oidtmann  on  the 
above  system.  The  same  person  has  also  employed  the 
process  to  produce  mosaic  glass  pictures,  similar  to 
the  mediaeval  glass  paintings  on  glass.  These  mosaic 
glass  pictures  are  produced  by  cutting  out  coloured 
pieces  of  glass  corresponding  to  the  figures  and  their 

*  See  "  Photographische.  Mittheilungen,"  Jahrg.  1869.  Berlin : 
Oppenheim. 


CHROMO -PHOTOGRAPHY.  259 

colours.    For  example,  for  a  human  figure,  the  outline  of 
the  face  was  drawn  on  a  flesh-coloured  glass  slab  and 
cut  out ;    the  same  thing  took  place  for  the  drapery,  on 
glass  slabs  corresponding  to  the  colours.    The  lights  and 
shades  and  details — for  example,  nose,  mouth,  and  eyes — 
were  then  drawn  with  black  moist  colour  on  the  proper 
piece  of  glass  assigned  to  it,  and  burnt  in,  after  which 
all  the  separate  pieces  of  the  glass  were  soldered  (or 
cemented)  together.     Dr.  Oidtmann  does,  by  means  of 
photography,  what  the  draughtsman  does  in  this  mosaic 
glass-painting.     He  copies  the  outlines  of  the  face  from 
the  large-sized    original    photograph,   or    the   original 
woodcut,  on  the  proper  piece  of  glass,  and  powders  it 
with  moist  black  paint,  and  he  thus  obtains  a  picture 
that  can  be  burnt  in,  and  which  can  be  treated  in  the 
manner  described.     At  the  Vienna  exhibition  there  was 
a  copy  of  "  The  Crucifixion"  by  Diirer,  produced  in  this 
manner,    and    composed    of    150    glass    pieces.       Dr. 
Oidtmann  prepares  the  large-sized  original  pictures  by 
magnifying  little    woodcuts    according   to    the    photo- 
graphic manner   (see  p.  95).      Dr.  Oidtmann  has  also 
attempted  to  produce  pyro-photographs,  by  proceeding 
on  the   principle   of   chromo -lithography   (see   p.   250, 
Photo-lithography).     He  copied  the  similarly  coloured 
parts  of  a  painted  drawing — covering  over  the  others — 
on  a  film  of  gum  and  chromium,  powdered  this  with  a 
suitable  colour,  and  then  copied  the  other  colours  of  the 
original  successively  in  the  same  manner.      He  thus 
obtained  a  powdered  picture,  which  was  then  burnt  in. 


260  THE    CHEMISTRY   OF   LIGHT. 

SECTION  X. — PHOTOGRAPHY  AND  THE  SAND-BLOWING  PROCESS. 

The  Nature  of  the  Sand-blowing  Process — Its  Connection  with  the 
Pigment  Press—  Its  Employment  in  Heliography  instead  of  Corrosive 
Acids. 

Tilghmann,  at  Philadelphia,  made  the  observation, 
during  his  residence  at  the  watering-place  Longbranche, 
that  the  windows  exposed  to  sea  wind  became  quickly 
misty.  He  found  that  this  was  occasioned  by  fine  sand, 
which  the  wind  drove  against  the  window;  this  gave 
him  the  idea  of  making  ground  glass  by  sand  blown  on 
to  it,  and  this  succeeded  perfectly.  He  covered  a  glass 
surface  with  an  iron  mould,  in  which  figures  and  letters 
were  cut ;  he  kept  this  in  a  current  of  air  bringing  sand 
with  it.  In  a  short  time  this  made  ground  glass  at  the 
places  that  were  exposed,  and  Tilghmann  obtained  thus 
a  drawing  of  the  incised  figures.  A  blast  of  only  four 
inches  hydraulic  pressure  and  a  period  of  ten  minutes 
are  required  for  this  work.  If  the  air  pressure  is 
stronger,  or  steam  is  used  instead,  conveying  sand, 
and  having  a  pressure  of  60  to  120  Ibs.  to  the  square 
inch,  the  effect  is  immense.  Sand  blown  with  such 
power  through  a  narrow  pipe  bores  deep  holes  into  the 
hardest  stones,  and  even  into  glass.  The  process  has 
been  used  to  bore  stone  and  metal  plates.  If  a  mould  of 
cast-iron  is  placed  on  it  in  which  the  figures  have  been 
cut,  they  can  be  deeply  engraved  in  a  short  time  in  the 
stone.  The  iron  plate  is,  no  doubt,  injured,  but  much 
more  slowly  than  the  stone  slab.  A  cast-iron  plate  -f^  of 
an  inch  thick  is  only  reduced  T^  of  an  inch,  whilst  a 
section  300  times  deeper  is  made  in  marble.  India-rubber 
endures  the  sand  stream  almost  as  well  as  iron.  You 
might  cut  into  marble  with  an  india  rubber-mould  200 


CHROMO -PHOTOGRAPHY.  261 

times  as  deep  as  the  depth  of  the  mould,  without  much 
injuring  it. 

With  the  pressure  of  100  Ibs.,  such  a  sand  stream 
can  penetrate  1J  inches  deep  into  granite,  4  inches  into 
marble,  and  10  inches  into  soft  sandstone. 

The  circumstance  that  soft  bodies  act  as  shields  in 
this  has  led  to  elegant  applications  of  this  method  in 
the  industrial  arts.  For  example,  if  glass  be  covered  with 
lace  pattern  and  a  sand  stream  take  effect  upon  it,  the 
glass  becomes  ground  in  the  meshes,  and  a  copy  of  the 
lace  is  obtained  on  glass.  In  the  same  manner,  you  can 
paint  with  gum  colour  upon  glass,  and  this  drawing 
can  b3  produced  clear  on  an  unpolished  ground  by  the 
sand  blast.  This  circumstance  led  immediately  to  the 
application  of  photography.  If  a  pigment  impression 
(p.  239) — that  is,  a  chromo-glucose  picture — is  produced 
on  glass  by  transferring  a  prepared  impression  directly  on 
glass  (see  above),  the  glass  surfaces  at  all  the  places  of 
the  picture  are  protected  by  a  layer  of  gelatine.  If  now 
a  sand  stream  is  allowed  to  operate  upon  it,  it  polishes 
the  glass  only  at  the  uncovered  places ;  thus  an  opaque 
and  transparent  glass  picture  is  the  result.  If  the  gela- 
tine picture  is  a  negative,  the  shadows  are  dim ;  and  such 
an  unpolished  slab  is  also  fit  for  giving  impressions  by 
means  of  printer's  ink.  Corroding  with  acids  often  eats 
into  the  fine  strokes  and  makes  them  broader.  In  place 
of  them  the  heliographic  metal  plates  of  Talbot  (p.  225) 
can  be  blown  upon  with  sand — which,  owing  to  its  per- 
pendicular position,  only  works  downwards — and  thus 
cavities  of  great  depth  can  be  produced,  so  that  plates 
thus  blown  upon  can  be  used  for  high  relief-printing, 
that  is,  book  printing.  Tilghmann  recommends  that  a 


262  THE    CHEMISTRY   OF   LIGHT. 

positive  of  gelatine  and  chromium  should  be  produced 
upon  a  cake  of  resin ;  that  this  should  be  blown  upon  and 
deeply  hollowed  out ;  then  a  form  is  obtained  which  can 
be  first  cast  in  gypsum,  and  then  in  metal  type,  which 
can  be  used  for  printing. 

These  are  interesting  experiments,  which  ought  in 
time  to  lead  to  important  practical  results. 

SECTION  XL — THE  PHOTOMETER  FOR  CHROMO-PHOTO  GRAPH  Y. 

IN  many  of  the  above  described  chromic  processes,  e.g. 
the  production  of  relief-prints,  pigment-prints,  light- 
prints,  etc.,  it  is  very  important  to  determine  the  exact 
time  for  exposure.  This  is  not  easy,  because  the  picture 


I? 


Fig.  94. 

appears  only  faint  or  not  at  all,  as  in  pigment-printing ; 
therefore  the  state  of  the  picture  gives  no  safe  criterion 
respecting  the  completion  of  the  picture.  This  circum- 
stance has  necessitated  the  application  of  a  photometer, 
easily  determining  the  duration  of  the  exposure.  Such 
photometers  have  been  provided  by  Byng  and  Swann  in 
England,  and  by  the  author.  The  author's  photometer 
consists  §f  a  semi-transparent  paper  scale  Z/,  whose 
transparency  diminishes  from  2  to  25.  (See  Fig.  94.) 
This  scale  is  formed  of  layers  of  paper,  whose  entire 


CHROMO-PHOTOGKAPHY.  263 

quantity  is  expressed  by  the  figure  printed  on  them  ; 
under  this  scale  is  exposed  a  strip  of  chromic  paper ; 
that  is,  paper  which  has  been  plunged  into  chromate  of 
potassium.  The  strip  is  enclosed  in  a  little  box  in  such 
wise  that,  when  the  cover  D  is  shut  down  with  the  scale, 
the  chromic  paper  and  the  scale  are  in  close  contact; 
the  light  now  shines  through  the  scale  and  browns  the 
paper  strips  lying  under  it.  This  colouring  affects,  first, 
the  thin  transparent  part  of  the  scale,  and  passes  thence 
to  the  opaque  end,  the  rapidity  depending  on  the  strength 
of  the  light.  To  know  how  far  the  effect  of  the  light  has 
extended,  figures  are  printed  on  the  scale  which  do  not 
permit  the  light  to  pass ;  therefore  these  remain  clear  on 
a  brown  ground,  and  the  place  to  which  the  effect  of 
light  has  advanced  is  perceived  by  the  figure  that 
appears  there. 

To  use  this  instrument,  some  experimental  copies 
must  be  made  first.  Supposing  it  were  desired  to  prepare 
a  pigment  impression  from  a  negative,  the  film  of  pig- 
ment is  exposed  under  the  negative  at  the  same  time 
with  the  photometer.  After  some  time,  lamplight  is 
used  to  see  how  far  the  photometer  paper  is  browned. 
The  significant  number  is  noted — photometer  degree — 
and  the  negative  is  only  half  covered,  the  other  half 
continuing  to  be  copied  until  a  higher  degree  of  the 
photometer.  Then  the  pigment  picture  is  developed, 
and  the  degree  of  the  photometer  is  determined  where 
the  favourable  result  has  been  obtained.  Rarely  more 
than  one  attempt  has  to  be  made ;  when  this  has 
determined  the  degree  up  to  which  the  negative  must  be 
copied,  the  time  of  exposure  can  always  be  regulated 
with  the  help  of  the  photometer.  Practised  hands  only 


264  THE    CHEMISTRY    OF   LIGHT. 

determine  the  degree  with  some  negatives,  and  easily 
ascertain  up  to  what  degree  a  fresh  negative  must  be 
copied. 

SECTION  XII. — THE  CHEMICAL  EFFECT  OF  LIGHT  AND  THE  PEA-SAUSAGE. 

In  the  campaign  of  1870,  the  well-known  pea-sausage 
was  one  of  the  most  important  articles  of  food  for  the 
army,  and  was  prepared  daily  in  many  thousands  of 
skins.  The  fabrication  of  the  interior  portion  caused  little 
difficulty,  but  the  obtaining  so  many  skins  created  much 
difficulty.  As  the  supply  fell  short,  a  substitute  was  sought 
in  parchment.  This  paper,  which  is  produced  by  dipping 
for  a  second  blotting-paper  in  sulphuric  acid,  then  wash- 
ing and  drying  it,  is  distinguished  by  its  skin-like 
properties  of  resistance.  It  is  impenetrable  to  water, 
and  difficult  to  tear.  It  is  therefore  used  for  the  pro- 
duction of  cheques  on  the  Treasury.  It  was  attempted 
to  fabricate  sausage  skins  with  this  paper,  by  doubling  a 
sheet  cylindrically  and  pasting  it  together.  No  glue  or 
gum  can  resist  the  effect  of  the  boiling  water  in  which 
the  sausage  has  to  be  cooked,  and  so  the  artistic  sausage 
skin  fell  asunder.  Dr.  Jacobson  solved  the  problem  by 
producing  an  adhesive  substance,  with  the  help  of  the 
chemical  effects  of  light,  which  could  resist  boiling 
water.  He  mixed  the  gelatine  intended  for  the  pea- 
sausage  skin  with  chromate  of  potash,  and  exposed  the 
adhesive  parts  to  the  light.  This  occasioned  the  insolu- 
bility of  the  gummy  substance,  and  now  the  artificial 
skin  endured  boiling  water  thoroughly  well.  The  number 
of  sausage  skins  prepared  in  this  way,  by  the  chemical 
operation  of  light,  amounted  to  many  hundred  thousands. 


CHAPTEE  XVI. 

IRON,  URANIUM,  AND   COPPER  PHOTOGRAPHY. 

Historical — Combinations  of  Iron — Effect  of  Ether  on  a  Solution  of 
Chloride  of  Iron — Chloride  of  Iron  and  Paper — Iron  Pictures  in  Blue 
— Iron-gold  Pictures — Pans  Process  with  Salts  of  Iron — Iodide 
Pictures — Combinations  of  Uranium— Uranium  Pictures — Their 
Development — Copper  Pictures  of  Obernetter. 

WE  remarked  further  back  that  the  number  of  sensitive 
substances  is  much  greater  than  appears,  and  a  close 
analysis  was  to  determine  that  all  bodies  were  more  or 
less  sensitive  to  light.  Even  in  the  first  period  of 
photography,  1840,  Herschel  observed  the  sensitiveness 
of  salts  of  iron,  Burnett  that  of  salts  of  uranium,  and 
Kratochvila  prepared  successful  daguerreotypes  on  copper 
plates,  in  a  manner  analogous  to  those  on  silver  plates. 
This  process  has  been  energetically  cultivated,  but 
hitherto  without  any  important  result. 

It  has  long  been  known  that  chloride,  of  iron,  a  yellow 
substance  consisting  of  iron  and  chlorine,  when  dis- 
solved in  ether  bleaches  in  the  light  and  becomes  the 
hypo-chloride  of  iron,  having  less  chlorine.  The  same 
thing  takes  place  in  connection  with  paper.  If  clean 
paper  is  saturated  with  a  solution  of  chloride  of  iron  in 
six  parts  of  water,  dried  in  the  dark,  and  exposed  under 


266  THE    CHEMISTRY   OF   LIGHT. 

a  negative  picture,  the  paper,  which  is  yellow  at  first, 
becomes  white  under  the  transparent  places,  because 
the  yellow  chloride  of  iron  passes  into  white  hypo- 
chloride  of  iron.  This  pale  hypo-chloride  picture  can 
be  easily  coloured  intensely  dark.  If  the  pale  picture 
is  plunged  in  a  solution  of  red  prussiate  of  potash,  this, 
combined  with  the  hypo-chloride  of  iron  reduced  by 
light,  easily  produces  Berlin  or  Prussian  blue,  while  it 
leaves  the  chloride  of  iron  unchanged ;  in  this  manner 
a  blue  picture  is  obtained.  If  a  pale  iron  picture  be 
plunged  in  a  solution  of  gold,  it  becomes  of  a  light  blue 
colour,  because  the  hypo-chloride  of  iron  produces  a  pre- 
cipitate of  metal  gold.  In  this  manner  a  dark  precipitate, 
which  is  produced  by  hypo-chloride  in  many  substances, 
will  give  a  dark  colour  in  all  such  cases. 

Another  process  is  the  transforming  of  iron  into 
iodide  pictures.  A  piece  of  paper,  saturated  with 
chloride  of  iron,  is  copied  under  a  positive  (for  example, 
a  drawing).  The  copy  comes  out  as  a  yellow  drawing 
of  an  unchanged  chloride  of  iron  on  a  white  ground.  If 
the  paper  be  now  plunged  in  a  solution  of  iodide  of 
calcium  and  starch,  the  iodide  becomes  liberated,  and 
forms  with  the  starch  a  dark  blue  iodide  starch,  which 
gives  a  strong  dark  shade  to  the  lines  that  were  originally 
pale  (Herschel). 

There  are  several  other  processes  to  make  iron 
pictures  of  a  darker  colour.  The  pictures  in  Berlin 
blue  do  not  hold,  becaus^  that  colour  turns  pale  in  the 
sun  (accordingly,  blue  parallels  rapidly  lose  their  colour 
in  the  light).  The  same  remark  applies  to  pictures  of 
iodide  of  starch ;  the  gold  pictures  are  too  pale  and  their 
restoration  too  costly. 


IRON,    URANIUM,    AND    COPPER   PHOTOGRAPHY.  267 

The  salts  of  uranium  present  the  same  phenomena  as 
these  salts.  Uranium  itself  is  a  rare  metal  whose 
combinations  play  a  great  part  in  colouring  materials; 
thus  there  is  a  yellow  oxide  of  uranium,  that  can  be 
burnt  into  porcelain,  giving  a  dark  green  colour,  and 
which,  being  mixed  with  glass,  imparts  to  it  a  beautiful 
grass-green  (Anna  glass).  Moreover,  a  chloride  of 
uranium  and  a  hypo-chloride  of  uranium  are  known, 
corresponding  to  the  chloride  of  iron  and  the  hypo- 
chloride  of  iron,  and  bearing  much  resemblance  to  the 
combinations  of  iron  just  mentioned.  The  most  noted 
salt  of  uranium  is  nitrate  of  uranium,  which  is  reduced 
to  sub-nitrate  of  uranium  by  the  influence  of  light  in  the 
presence  of  organic  bodies — for  example,  paper  recepta- 
cles. If  a  piece  of  paper  is  dipped  in  a  solution  of 
one  part  of  this  salt  and  five  parts  of  water,  if  it  be  then 
dried  in  the  light  under  a  negative,  a  very  faint,  scarcely 
perceptible  picture  is  obtained,  consisting  of  sesqui- 
oxide  of  uranium.  If  this  is  plunged  in  a  solution  of 
silver,  or  a  solution  of  gold,  it  becomes  suddenly  visible, 
because  the  sesqui-oxide  of  uranium  precipitates  directly 
the  gold  or  silver  metal  as  a  coloured  powder  (in  the 
case  of  the  silver,  brown ;  and  in  that  of  gold,  violet). 

The  uranium  is  too  rare  and  too  dear  to  be  employed 
generally  in  photography. 

As  can  be  perceived,  the  salts  of  iron  and  the  salts  of 
uranium  are  analogous  to  the  salts  of  chromium,  by 
only  being  sensitive  to  light  in  the  presence  of  organic 
bodies.  In  a  pure  state,  salts  of  uranium  and  salts  of 
iron  do  not  change  in  the  light. 

The  sensitiveness  to  light  of  salts  of  copper  has 
hitherto  only  been  studied  very  imperfectly.  Copper 


268  THE    CHEMISTRY   OF   LIGHT. 

forms  with  chlorine  a  green  salt,  soluble  in  water, — 
chloride  of  copper, — which  is  reduced  to  hypo-chloride 
of  copper  in  the  light.  Obernetter  took  advantage  of 
this  fact,  mixing  chloride  of  copper  and  chloride  of  iron 
together,  and  saturating  the  paper  with  them.  This 
was  exposed  to  light  under  a  negative,  then  plunged  in 
sulpho-cyanide  of  potassium,  and  ultimately  treated  with 
red  prussiate  of  potash.  The  result  produced  by  this 
somewhat  complicated  process  was  a  brown  picture.* 

*  See  Yogel, " Lehrbucb.  der  Photographic,"  p.  32.     Berlin:  Oppcn- 
lieim. 


CHAPTEE  XVIL 

THE  CHANGE  OF  GLASS  UNDER  THE  INFLUENCE  OF  LIGHT. 

Faraday's  Observation  on  Manganese-glass — Change  of  Mirror-glass  in 
the  Light — Almost  all  kinds  of  Glass  are  Sensitive  to  Light — 
Gaffield's  Experiments — Disadvantages  of  the  Change  of  Glass  in 
the  Light — Explanation  of  the  Change  of  Manganese-glass — 
Operation  of  Light  on  Topaz. 

THE  celebrated  natural-philosopher  Faraday  made  the 
observation  that  glasses  painted  with  manganese,  and 
conspicuous  for  a  peculiar  flesh  tint,  became  rapidly 
brown  in  the  light.  This  fact  remained  for  a  long  time 
without  further  results.  But  some  years  later,  other 
observations  of  the  same  kind  were  made. 

A  very  handsome  plate  of  glass  was  exhibited  in  a 
mirror  shop  at  Berlin.  It  bore  the  inscription  "  Manu- 
facture of  Mirrors  "  in  brass  letters.  After  being  exhibited 
for  years,  the  business  was  broken  up,  and  the  mirror, 
on  account  of  its  beauty,  was  takere  away  by  its 
proprietor,  the  brass  letters  were  effaced,  and  the  plate 
was  cleaned.  To  the  surprise  of  the  proprietor,  the 
letters  remained  plainly  visible  on  the  glass,  notwith- 
standing all  attempts  to  remove  them.  The  surface  was 
even  abraded,  but  this  did  not  produce  any  effect  on  the 
letters.  It  was  found  that  the  glass  was  penetrated 


270  THE    CHEMISTRY   OF   LIGHT. 

with  yellow  marks,  and  that  it  remained  white  only  at 
the  places  where  the  opaque  letters  had  kept  off  the 
light.  The  plate  of  glass  was  afterwards  cut  into  two 
halves.  One  half,  with  the  word  "  mirror,"  remains  in 
the  hands  of  the  Philosophical  Collection  of  the  Uni- 
versity of  Berlin.  Gaffield  has  lately  made  very  interest- 
ing observations  on  the  change  of  glass  in  the  light,  and 
has  thus  determined  that  almost  all  kinds  of  glass  are 
sensitive  to  the  light,  and  that  often  an  exposure  of  only 
a  few  days  suffices  to  effect  this  change. 

Gaffield  went  systematically  to  work  in  his  experi- 
ments. He  cut  the  glass  in  question  into  two  parts,  placed 
one  in  the  dark  and  the  other  in  the  light,  and  compared 
the  two  after  a  few  days.  In  almost  all  cases  he 
remarked  a  darkening  of  the  colours.  Only  two  kinds 
of  greenish  German  and  Belgian  window-glass  remained 
unchanged.  The  glasses  of  a  darker  colour  lost  their 
colour  when  exposed  to  red  heat. 

This  change  of  glass  in  the  light  has  a  very  unfavour- 
able effect  in  photographic  studios.  Through  the 
yellowish  colouring  which  their  glass  assumes,  in  time 
a  part  of  the  chemically  operative  light  is  absorbed  in 
the  glass.  The  deterioration  of  light  thus  resulting 
makes  itself  remarked  in  a  very  conspicuous  manner, 
because  the  time  which  is  necessary  in  order  to  take  a 
portrait  must  be  continually  lengthened. 

The  glasses  that  contain  manganese  change  most 
strikingly.  Hyper-oxide  of  manganese,  also  named 
brownstone,  is  often  added  to  glass  to  discolour  it. 
The  dark  green  sesqui-oxide  of  iron  in  the  glass  is 
transformed,  by  the  operation  of  the  oxygen  of  the 
brownstone,  into  the  paler  protoxide  of  iron,  and  the  dis- 


CHANGE    OF   GLASS   UNDER   INFLUENCE    OF   LIGHT.      271 

colouration  is  effected  in  this  manner.  The  opposite 
effect  takes  place  in  the  light.  The  oxide  of  iron  is 
reduced  again  to  sesqui-oxide  of  iron ;  the  oxygen  passes 
to  the  manganese,  and,  forming  brown  oxide  of  man- 
ganese, gives  rise  in  this  manner  to  the  dark  colouring. 
In  many  minerals,  light  has  an  opposite  effect  to  that 
which  it  has  on  glass.  It  does  not  colour  them,  but 
discolours.  This  happens  especially  with  the  Siberian 
topaz,  which  soon  loses  its  golden-yellow  colour  in  the 
light.  A  splendid  crystal  of  topaz,  six  inches  high, 
belonging  to  the  Mineralogical  Museum  at  Berlin,  has 
in  this  manner  lost  materially  in  the  beauty  of  its 
appearance. 


272  THE    CHEMISTRY   OF   LIGHT. 


CHAPTEE  XVIII. 

PHOTOGRAPHY  IN  NATURAL  COLOURS. 

Observation  of  Seebeck  and  Herschel — Bequerel's  Painted  Pictures  and 
Silver  Plates — Niepce's  Labours — Effect  of  Black  Colours — Coloured 
Pictures  on  Paper  of  Poitevin  and  Zencker — Want  of  a  Fixing 
Medium  for  Coloured  Photographs. 

PHOTOGRAPHY  has  already  achieved  grand  results;  but 
it  has  still  one  problem  to  solve — the  production  of 
photographs  in  natural  colours.  There  are  plenty  of 
coloured  photographs  to  be  seen,  but  in  such  cases  the 
colour  has  been  added  after  by  the  paint  brush ;  it  is  a 
kind  of  retouche,  which  in  most  cases  does  not  improve 
the  picture.  But  we  are  now  speaking  of  photographs  in 
natural  colours,  reproducing  the  original  colours  of 
objects  solely  and  alone  by  the  operation  of  light. 
Numerous  attempts  are  at  hand,  pointing  more  or  less 
to  this  great  end.  The  production  of  coloured  pictures, 
by  the  chemical  effect  of  light,  has  been  successfully 
achieved ;  but  these  are  .spoiled  soon  by  the  influence  of 
the  same  agent  to  which  they  are  indebted  for  their 
production.  No  means  exist  at  present  of  fixing  coloured 
photographs. 

The  first  attempts  to  make  coloured  pictures  date  a 


PHOTOGRAPHY  IN   NATURAL   COLOURS.  273 

long  way  back.  Professor  Seebeck  of  Jena,  as  early  as 
1810,  found  that  chloride  of  silver  took  in  the  colour 
spectrum  almost  the  same  colour  corresponding  to 
them.  This  observation,  published  in  Goethe's  "Farben- 
lehre,"  II.,  page  716,  passed  unnoticed.  Only  since  1841, 
after  the  discovery  of  the  daguerreotype,  the  noted  Sir 
John  Herschel  made  experiments  in  the  same  direction. 
He  took  paper  saturated  with  chloride  of  silver  and 
nitrate  of  silver,  let  a  powerful  solar  spectrum  fall  upon 
it,  and  obtained  immediately,  like  Seebeck,  a  coloured 
image  of  the  spectrum,  agreeing,  however,  only  imper- 
fectly with  the  original.  Bequerel  was  more  successful. 
He  ascertained  that  the  solution  of  nitrate  of  silver  in 
Herschel's  experiments  had  a  disturbing  effect,  and  he 
worked  with  chloride  of  silver  alone.  He  employed 
silver  plates,  which  he  plunged  in  chlorine  water.  The 
plates  become  thus  whitish,  by  the  formation  of  chloride 
of  silver,  and,  when  exposed  to  the  spectrum,  present  a 
picture  whose  colours  agree  very  nearly  with  the  natural 
ones.  Bequerel  observed  that  the  duration  of  the 
operation  of  the  chlorine  water  was  very  important,  and 
he  preferred  at  a  later  date  to  chlorize  the  plates  by  the 
operation  of  the  galvanic  current.  To  this  end  he  sus- 
pended them  to  the  copper  pole  of  a  galvanic  battery  (see 
p.  227)  and  plunged  them  in  salt  acids.  The  galvanic 
current  decomposed  these  acids  into*  chlorine  and 
hydrogen.  The  chlorine  passes  to  the  silver  plate,  and 
forms  chloride  of  silver.  This  method- enables  the 
operator  to  produce  a  film  of  chloride  of  silver  of  any 
thickness,  according  to  the  duration  of  the  operation  of 
the  electric  current.  The  brownish  hypo-chloride  of 
silver  is  thus  produced,  and  this  is  chiefly  sensitive  to 


274  THE    CHEMISTRY   OF   LIGHT. 

colour.  Yet  this  sensitiveness  is  not  great,  it  only 
suffices  to  fix  a  powerful  spectrum,  but  it  requires  a  very 
long  exposure  to  obtain  images  from  the  camera 
obscura,  and  all  such  images,  unfortunately,  darken 
through  the  continuous  operation  of  light.  Bequerel 
found  that  the  sensitiveness  was  increased  by  heating 
the  plates.  This  observation  was  tunned  to  account  by 
his  successor,  Niepce  de  St.  Victor  (the  nephew  of 
Nicophore  Niepce,  see  p.  9).  This  savant  made  numer- 
ous experiments  from  1851-67  to  produce  coloured 
photographs,  and  he  imparted  his  observations  to  the 
Paris  Academy. 

He  worked,  like  Bequerel,  with  silver  plates,  which  he 
chlorized  by  plunging  in  a  solution  of  chloride  of  copper 
and  chloride  of  iron,  then  heated  to  a  high  degree,  and 
thus  obtained  plates  which  appeared  ten  times  more 
sensitive  than  Bequerel's,  and  allowed  him  to  copy  in  the 
camera  obscura  copper  lithographs,  flowers,  church 
windows,  etc.  He  relates  that  he  not  only  obtained 
colours,  but  that  gold  and  silver  remained  in  their 
metallic  splendour  in*  pictures,  and  the  picture  of  a 
peacock's  feather  retained  the  lustre  of  nature. 

Niepce  de  St.  Victor  introduced  a  further  improve- 
ment, by  covering  the  plate  of  chloride  of  silver  with  a 
peculiar  varnish,  consisting  of  dextrine  and  a  solution  of 
chloride  of  lead.  This  coating  made  the  plates  still 
more  sensitive  and  durable.  At  the  Paris  exhibition  of 
1867,  Niepce- exposed  the  different  coloured  photographs 
that  lasted  above  a  week  in  a  subdued  daylight  (they 
were  exposed  in  half-closed  boxes). 

Among  these  pictures  were  a  couple  of  uncoloured, 
but  perfectly  black  pictures  on  a  white  ground.,  which 


PHOTOGRAPHY  IN   NATURAL   COLOURS.  275 

had  been  copied  from  copper-plate  engravings.  These 
excited  great  interest,  and  justly  so,  because  in  these 
pictures  the  darkest  influence  had  had  most  effect,  and 
the  cleanest  and  whitest  the  least.  This  was,  therefore,  a 
directly  contrary  effect  to  what  happens  on  photographic 
paper,  where  the  dark  produces  a  clear  effect,  and  the 
clear  a  dark  effect  (see  p.  28).  This  production  of  black 
by  black  can  only  be  explained  by  assuming  that  the 
black  is  actually  not  black,  but  that  it  irradiates  ultra- 
violet light  invisible  to  the  eye  (see  p.  60). 

Since  Niepce,  who  died  in  1870,  the  only  persons  who 
have  directed  attention  to  coloured  pictures  are  Poitevin 
at  Paris,  Dr.  Zencker  *  at  Berlin,  and  Simpson  in 
London.  But  the  two  former  investigators  have  re- 
verted to  the  older  process,  as  employed  by  Seebeck  and 
Herschel,  i.e.  they  prepared  pictures  on  paper  again, 
only  the  preparation  of  this  p^per  was  peculiar.  Paper 
saturated  with  salts  was  made  sensitive  in  a  solution  of 
silver,  like  the  photographic  positive  paper  (see  p.  50), 
then  washed  to  remove  the  solution  of  silver,  and  after- 
wards exposed  to  the  light  in  a  solution  of  hypo-chloride 
of  tin.  By  this  means  violet  hypo-chloride  of  silver  is 
formed  from  the  white  chloride  of  silver.  The  hypo-chloride 
of  tin  only  operates  as  a  reducing  medium.  This  paper 
is  in  itself  little  sensitive  to  colouring ;  but  if  it  be  treated 
with  a  solution  of  chromic  acid  of  nifae  and  copper 
vitriol,  its  sensitiveness  increases  considerably,  so  that 
it  is  easy  to  copy  with  it  pictures  of  transparent 
colours.  Nevertheless,  the  colours  are  never  so  vivid  as 
in  the  original,  the  red  tones  showing  themselves  the 

*  Those  who  take  a  special  interest  in  the  matter  are  referred  to 
Dr.  Zencker 's  "  Lehrbuch  der  Photochromie."     Berlin,  1868. 


276  THE    CHEMISTRY   OF   LIGHT. 

strongest.  After  copying,  the  pictures  are  washed  with 
water,  to  make  them  less  sensitive  to  light.  In  this 
condition  they  showed  tolerably  well  in  a  subdued  light, 
but  no  means  have  been  found  hitherto  to  make  them 
perfectly  durable.  The  fixing  natrium  of  the  photo- 
grapher (see  p.  27)  cannot  be  employed,  as  it  destroys  the 
colours  directly.  We  must  hope  that  future  investi- 
gators will  succeed  in  supplying  this  deficiency.  The 
first  attempts  in  uncoloured  photography  also  failed 
for  want  of  a  fixing  medium  (see  p.  6),  which  was  only 
discovered  seventeen  years  later  by  Herschel, 


CHAPTEE  XIX. 

PHOTOGRAPHY  AS  A   SUBJECT   TO   BE   TAUGHT  IN  ART 
AND   INDUSTRIAL    SCHOOLS. 

Importance  of  School  Photography — Its  Use  for  Technical  Institutions — 
Photography  as  an  Object  to  be  taught  in  Art  Schools  and  Univer- 
sities. 

THE  previous  chapters  prove  how  manifold  are  the 
applications  of  photography.  It  has  entered  into  art, 
science,  industry,  and  life  as  a  new  kind  of  written 
language.  Photography  is  to  appearance  what  printing 
is  to  thought.  Typography  multiplies  what  is  written, 
photography  what  is  drawn ;  nay,  more,  it  also  draws  in 
a  chemical  way.  No  doubt  a  certain  technical  practice 
of  the  art  is  required  that  can  only  be  gained  by  experi- 
ence ;  but  it  is  easy  to  learn,  and  the  time  cannot  be 
distant  when  it  will  be  taught  as  an  extension  of  drawing 
— itself  a  matter  of  tuition — in  all  industrial  schools. 
Years  are  devoted  to  the  study  of  the  art  of  drawing,  of 
piano  playing,  and  other  things;  a  course  of  instruc- 
tion lasting  half  a  year  would  suffice  to  learn  photo- 
graphy. 

The  author  has  for  nine  years  presided  over  a  pro- 
fessor's  chair  of  photography  in  the   Eoyal  Industrial 
13 


278  THE    CHEMISTRY   OF  LIGHT. 

Academy  at  Berlin,  the  only  technical  institution  in 
Germany  where  photography  has  as  yet  obtained  a 
footing  in  the  curriculum.  It  is  by  no  means  the  object 
of  this  institution  to  train  professional  photographers; 
it  only  admits  photography  so  far  as  it  has  importance 
for  industry  and  science. 

At  this  institution  practical  exercises  are  carried  out 
in  the  positive  and  negative  processes  of  photography, 
especially  in  its  application  to  reproduce  drawings,  for 
taking  machines  and  buildings ;  and,  moreover,  instruc- 
tions are  given  on  the  licht-paus  process.  Other  technical 
institutions  are  still  hesitating  about  admitting  photo- 
graphy. The  importance  of  the  matter  is  still  depre- 
ciated, and  what  is  new  is  still  viewed  by  many  as 
inconvenient.  We  cannot  avoid  introducing,  in  this 
connection,  a  passage  from  a  work  that  has  recently 
appeared:  " Photography  as  a  Matter  for  Teaching  in 
Schools  of  Industry,"  by  Professor  Krippendorf,  of 
Arau.  The  author  remarks : — 

"  Schools  of  industry  are  instituted  with  the  special 
view  to  prepare  pupils  for  the  subsequent  professions  of  a 
technical  and  industrial  life,  and  therefore  they  natur- 
ally admit  in  their  curriculum  the  arts  and  sciences 
devoted  to  this  end,  especially  drawing.  Industrial 
training  must  see  not  only  that  these  branches  form  an 
organic  whole,  taking  the  place  of  the  old  classical 
languages  as  a  basis  of  general  culture;  it  must  also 
draw  into  the  province  of  science  the  new  inventions 
introduced  in  industry,  and  then  suffer  them  to  work 
back  on  practical  pursuits  in  a  more  profitable  manner.1 

"Photography  is  one  of  the  branches  that  has  advanced 
most  rapidly  in  the  last  ten  to  twenty  years.  It  is  a 


PHOTOGRAPHY   IN   ART    SCHOOLS.  279  - 

genuine  product  of  natural  science,  not  as  the  mere  play- 
thing of  accidents,  but  having  the  great  merit  of  being 
first  conceived  as  an  ideal,  and  then  practically  carried 
out.  It  is  therefore  an  art  full  of  value  in  itself,  based 
on  science,  and  one  whose  productions  delight  and  are 
gladly  viewed  by  all,  extending  the  knowledge  of  pupils 
and  giving  an  idealizing  tendency  to  young  minds. 

"There  is  scarcely  any  other  branch  in  industrial 
schools  in  which  it  is  a  downright  necessity  to  keep  in 
view  an  independent  observation  of  the  result.  Physics 
and  chemistry  are  taught  as  successful  results,  and  give 
no  clue  to  detect  the  source  of  errors.  The  pupil  only 
observes  what  the  teacher  puts  before  him,  and  both  are 
satisfied  if  the  law  is  found  in  the  experiments.  A 
learning  how  to  observe  does  not  properly  take  place, 
yet  this  is  specially  fitted  to  sharpen  the  judgment.  But 
if  photography  is  admitted  in  the  school,  we  gain  a 
branch  which  fixes  the  attention  of  the  pupil  in  a  way 
that  no  other  can  do.  The  study  of  photography  is 
specially  based  on  the  avoidance  of  sources  of  error,  and 
consists  in  the  necessity  of  setting  aside  certain  dis- 
advantages and  treating  their  source ;  hence  it  is  en- 
titled, when  the  art  is  properly  appreciated,  to  be 
introduced  in  all  such  schools. 

"Many  other  grounds  can  be  noticed  in  favour  of 
this  introduction. 

"Art  and  science  are  learnt  in  technical  schools  for 
practical  ends.  Practice  and  a  knowledge  of  drawing 
are  specially  demanded  on  entering  the  engineering 
profession.  It  may  even  be  affirmed,  of  two  equally 
talented  pupils,  the  best  draughtsman  will  take  the  first 
place.  Drawing  is  the  centre  of  gravity  for  most  techni- 


,  280  THE    CHEMISTRY   OF   LIGHT. 

cal  professions,  and  for  this  reason  alone  it  ought  not 
to  be  neglected  to  promote  technical  and  freehand 
drawing  in  every  direction.  But  photography  is  destined 
to  support  these  technical  studies,  as  it  is  also  a  mode 
of  drawing.  Indeed,  if  it  be  proposed  to  draw  a  compli- 
cated machine— as  a  weaver's  wheel — in  a  few  minutes, 
photography  is  the  only  means  to  do  this.  The  labour, 
otherwise  requiring  weeks,  is  reduced  to  an  affair  of  a 
quarter  of  an  hour,  and  is  so  perfectly  done  that  all  pro- 
portions are  duly  observed,  and  the  projection  must  be 
correct  from  whatever  point  it  is  taken,  if  proper  lenses 
be  used. 

"  If  we  follow  the  biography  of  gifted  pupils,  we  often 
trace  them,  aided  by  Government  stipends,  going  first 
on  distant  journeys  to  study  modern  and  ancient  monu- 
ments, and  to  bring  home  as  faithful  designs  of  them  as 
possible.  What  a  severe  labour  this  implies  for  the 
architect,  amidst  a  foreign  population,  in  a  trying 
climate,  who  has  to  project  faithful  sketches  in  a  short 
time  amidst  countless  obstacles !  On  the  other  hand, 
how  it  is  all  abridged  by  photography!  "What  would 
not  the  young  travelling  engineer  give  to  take  plans  of 
entire  manufactories  which  he  has  only  a  few  minutes  to 
view  ?  What  would  not  the  highly  cultivated  philologist 
give  to  retain  for  himself  and  others,  in  a  durable  form, 
the  overpowering  impressions  of  life  in  the  past,  which 
he  can  only  feel  as  a  transient  emotion,  on  the  classical 
ground  of  Greece  and  Italy  ?  It  is  our  duty  to  announce 
it  publicly,  that  all  these  wishes  have  become  a  possible 
and  a  tangible  reality  through  the  progress  of  photo- 
graphy, and  that  the  practice  necessary  to  effect  this  is 
easily  attained. " 


PHOTOGRAPHY  IN  ART  SCHOOLS.          281 

Krippendorf  omits  here  an  important  .point,  which  is 
the  great  value  of  photography  to  those  who  are  devoted 
to  practical  typography,  whether  it  be  lithography,  book 
printing,  copper-plate  printing,  printing  of  paper  money, 
porcelain  manufacture  or  dyeing ;  for  in  all  these  branches 
the  aid  of  photography  is  very  important.  In  this  con- 
nection we  refer  to  the  chapter  on  pyro-photography, 
heliography,  and  chromo-photography.  In  these  branches 
we  see  photography  an  auxiliary  to  the  multiplying  arts. 

Though  it  has  done  great  things  in  this  connection, 
we  see  very  few  heliographers  and  photo-lithographers. 
The  ground  of  this  is  found  simply  in  the  fact  that 
art  schools,  training  lithographers  and  copper-plate 
engravers,  entirely  overlook  photography.  It  is  set 
aside  as  no  art  at  all  by  persons  who  feel  themselves 
artists,  yet  to  whom  it  would  be  useful.  But  in  the 
before-mentioned  alliance  of  photography  with  litho- 
graphy and  metal-plate  printing,  good  results  can  only 
be  achieved  by  the  operator  being  equally  skilled  in  both 
arts.  The  author  has  often  witnessed  the  failure  of 
heliographers,  lithographers,  and  photographers  who 
tried  to  work  by  combining  the  two  arts.'  It  is  therefore 
necessary  that  the  schools  of  art  should  take  the  matter 
in  hand,  and  if  so,  a  new  province,  hitherto  unknown  to 
the  lithographer,  that  of  light-printing  (see  p.  243), 
must  soon  become  domiciliated  in  those  institutions. 

But  a  knowledge  of  photography  is  equally  important 
for  painters.  Photography  copying  oil-paintings  has 
taken  a  magnificent  development  in  our  time.  Adverse 
opinions  to  it  are  indeed  uttered  by  stiff  art  critics, 
such  as  Thansing,  just  as  idealistic  tourists  formerly 
ranted  against  railways,  because  travel  was  thereby 


282  TEE    CHEMISTRY   OP  LIGHT. 

robbed  of  its  poetry.  These  people  were  right  from 
their  point  of  view,  but  they  could  not  stop  the  intro- 
duction of  railways ;  and,  though  travel  may  have  become 
less  poetic  through  them,  these  lines  have  the  advantage 
of  giving  an  opportunity  to  persons  of  slender  means  to 
make  excursions,  and  thereby  to  enrich  their  minds  with 
a  knowledge  of  foreign  countries  and  people,  and  of 
improving  their  health.  Photography  affords  to  persons 
of  small  income  similar  advantages  in  the  province 
of  art.  Paintings  too  expensive  to  be  purchased  by  any 
save  the  rich,  became  slowly  and  imperfectly  known  to 
others  by  the  expensive  medium  of  copper-plate  prints. 
These  engravings  were  also  confined  to  a  limited  circle. 
But  now  photography  reproduces  with  the  rapidity  of 
lightning,  and  with  all  faithfulness,  the  latest  creations 
of  art,  and  thus  its  cheapness  makes  them  accessible  to 
all.  The  copy  is  not  so  artistic  as  that  of  the  engraver, 
but  it  suffices  to  bring  all  that  is  new  quickly  to  the 
knowledge  of  all,  and  in  spite — or  in  consequence  rather 
• — of  this,  the  engraving  coming  after  still  retains  its 
value. 

Moreover,  the  negatives  from  oil-paintings  require 
working  up  by  retouche,  in  order  to  equalize  the  false 
effects  of  colouring.  This  retouche  may  be  very  injurious 
if  carried  out  by  unskilled  hands.  The  most  suitable 
hand  is  that  of  the  painter  who  painted  the  original. 
Good  painters  have  already  successfully  worked  at 
reproducing  negatives  from  their  own  originals,  and  the 
impressions  from  plates  touched  up  by  the  artists  them- 
selves must  evidently  have  a  much  higher  value  than 
those  emanating  from  other  hands.  This  presents  a 
fine  new  field  for  the  artist ;  but  it  can  only  be  worked 


PHOTOGRAPHY  IN  ART  SCHOOLS.         283 

with  good  results  if  the  art  pupils  have  become  familiar 
in  art  schools  with  the  technical  routine  of  negative 
retouche,  and  with  taking  positive  impressions  connected 
with  them. 

In  conclusion  we  add  a  few  words  on  the  development 
of  the  professional  photographer. 

We  have  already  shown  that  if  portrait  and  landscape 
photography  are  to  produce  really  solid  results,  they 
require  a  knowledge  of  the  principles  of  art.  But 
hitherto  nothing  has  been  done  to  train  photographers 
artistically.  Moreover,  photography  can  only  be  raised, 
in  an  artistic  point  of  view,  when  art  schools  render  a 
study  of  art  possible  to  the  photographer.  The  time 
must  be  at  hand  when  all  small  jealousy  directed  against 
photography  must  fall  to  the  ground.  Experience  has 
already  found  that  it  is  not  a  rival,  but  a  handmaid  to 
the  solidly  trained  artist. 

Photography  can  be  the  more  readily  introduced  in 
schools,  as  its  tuition  requires  much  less  time  than 
drawing,  and  with  better  results.  Four  hours  a  week  for 
six  months  suffice  to  train  a  pupil  enough  in  photo- 
graphy to  enable  him  to  go  on  by  himself,  even  without 
a  knowledge  of  chemistry. 

Schools  of  science,  as  well  as  of  art,  must  attend  to 
this  branch,  because  photography  is  very  useful  as  an 
aid  to  natural  science. 

This  new  art  gives  beautiful  illustrations  for  science 
and  art  lectures  by  the  magic  lantern.  The  investigator 
can  by  its  means  give  faithful  original  pictures  of  the 
results  of  his  labours  (see  page  93).  Hitherto  proper 
apparatus  was  wanted;  the  magic  lanterns  sold  in 
Germany  gave  imperfect  images.  Latterly,  E.  Talbot  in 


284  THE    CHEMISTRY   OF   LIGHT, 

Berlin,  has  introduced  American  magic  lanterns,  which 
are  best  adapted  for  lectures. 

The  Woodbury  press  has  been  joined  to  the  above,  for 
the  illustration  of  lectures ;  and  the  latest  improvements 
in  dry  plate  photography  have  had  the  result  that  dry 
plates,  like  licht-paus  paper,  have  become  articles  of 
trade,  making  the  production  of  photographs  much  more 
easy.  Thus  one  improvement  combines  with  another  to 
make  photography  what  it  ought  to  be — a  universal  art 
of  writing  by  light ! 


AMERICAN    APPENDIX. 


THERE  is  an  omission  in  the  foreg-oing  work,  which,  though 
it  may  matter  little  with  the  European  editions,  it  is  worth 
while  to  point  out  in  the  American  edition  :  the  distinguished 
author  has  hardly  done  justice  to  the  science  of  this  country 
in  its  contributions  to  the  development  of  his  subject. 

Professor  Vogel  ascribes  the  first  daguerreotypes  to  Pro- 
fessor Morse;  and  states  that  "his  coadjutor  was  Professor 
Draper."  Now,  the  fact  is,  that  Dr.  J.  W.  Draper,  of  New 
York,  was  the  first  person  who  took  daguerreotype-portraits 
from  the  life,  which  was  in  September,  1839.  He  published 
an  announcement  of  it  in  the  London  and  Edinburgh  Phil- 
osophical Magazine  of  March,  1840,  and  shortly  afterward 
gave  a  detailed  account  of  the  whole  operation  in  the  same 
journal.  Dr.  Draper  had  been  already  for  a  considerable 
time  making  researches  on  the  chemistry  of  light,  and  was 
not  only  familiar  with  the  whole  subject,  but  was  active  in 
its  original  exploration.  In  his  laboratory  in  the  New  York 
University,  Professor  Morse  learned  the  art. 

Dr.  Draper  was  also  the  first  to  photograph  the  fixed  lines 
of  Fraunhofer  in  the  solar  spectrum,  and  to  show  that  they 
exist  in  large  numbers  beyond  the  violet  space.  ^ 

He  was,  besides,  the  first  to  decompose  carbonic  acid  in  the 
actual  spectrum  ;  and  to  prove  that  it  is  the  yellow  light  that 
is  efficient  in  this  change,  and  not  the  violet  rays,  as  had  been 
formerly  supposed. 


APPENDIX. 

As  has  been  recognized  by  high  foreign  authorities,  and  as 
is  well  known,  Dr.  Draper  was  the  first  to  devise  a  method 
for  measuring  the  intensity  of  the  chemical  rays,  by  which  it 
first  became  possible  to  subject  them  to  quantitative  investi- 
gations. 

He  was,  moreover,  the  founder  of  astronomical  photogra- 
phy, having  taken  pictures  of  the  moon,  which  were  exhibited 
at  the  New  York  Lyceum  of  Natural  History,  March  23,  1840. 

Dr.  Draper  also  established  the  great  principle  which  is.  at 
the  basis  of  spectrum  analysis.  In  a  memoir  "  On  the  Pro- 
duction of  Light  by  Heat,"  he  showed  experimentally  that  all 
solid  substances,  and  probably  liquids,  become  incandescent 
at  the  same  temperature  of  about  977°  Fahr. ;  that  the  spec- 
trum of  an  incandescent  solid  is  continuous,  and  contains 
neither  bright  nor  dark  fixed  lines;  that  from  common  tem- 
peratures up  to  977°  Fahr.,  the  rays  emitted  by  a  solid  are 
invisible,  but  at  that  temperature  they  impress  the  eye  with 
the  sensation  of  red  ;  that  as  the  heat  of  the  body  continuous- 
ly rises,  other  rays  are  added,  increasing  in  refrangibility  as 
the  temperature  ascends.  The  English  professor,  Roscoe,  in 
the  third  and  last  edition  of  his  "  Spectrum  Analysis,"  ac- 
knowledges that  "  this  law  was  discovered  by  Draper  ; "  but 
the  German,  Kirchhoff,  although  he  had  himself  built  upon 
Draper's  results,  makes  no  reference  to  them  in  his  historical 
sketch  of  spectrum  analysis. 

The  author  of  the  present  work  touches  upon  these  various 
points,  but  is  careless  of  what  Dr.  Draper  has  done — an  over- 
sight which  is  the  more  marked,  as  he  seems  careful  to  ac- 
knowledge the  claims  of  savants  in  the  different  countries  of 
Europe.  E.  L.  Y. 

NEW  YORK,  April,  1875. 


INDEX. 


ACCURACY  of  Photographs,  120 
Aden,  Eclipse  of  the  Sun,  175 
^Esthetic  Defects  of  Photographs, 

21 

Albert,  243 
Albert-type,  244 
Albumen   Pictures    (see  White   of 

Egg)  32 

Aniline  Press,  246 
Art  and  Photography,  277 
Artificial  Light  and  Photography, 

68 

Asphalt,  9 
Astronomical  Photography,  171 

BANK-NOTES  copied  by  Photogra- 
phy, 11 

Barometrical  Observations  by  Pho- 
tography, 200 

Becquerel,  273 

Berkowsky,  174 

Brewster,  99 

Bromine,  109 

Buildings,  control  of,  by  Photo- 
graphy, 220 

Burchard,  254 

CABINET-SHAPE,  53 

Camera,  7,  89 

Carbonic  Acid,  Decomposition   of 

Light  by,  76 
Cartes  de  Visite,  53 


Charcoal  Pictures,  239 

Chemical  Ink,  5 

China  Decorations,  Photographic, 
213,  257 

Chloride  of  Silver,  4,  110 

Chromic  Oxide,  222 

Chromic  Photography,  221 

Chromium,  222 

Collodion,  34 

Contrasts  of  Light  and  Shade  re- 
produced  in  Photography,  126 

Copper-plate  Printing,  Photogra- 
phic, 10,  215,  226 

Copper  Salts  in  Photography,  267 

Copying  Frames  26 

Corona,  Photography  of  the,  185 

DAGUERUE,  12 
Daguerreotype,  13,  14 
Davy,  5 

Development;  17,  42,  113 
Disderi,  36 
Distance,  142 
Draper,  17 
Dubroni,  2C8 

EGG,  White  of,  33 
Electric  Light,  71 
Elements,  107 
Encaustics,  206, 255 
Ettinghausen,  16 


286 


INDEX. 


FACSIMILE,  241 

Field  Photography,  16G 

Fixing,  24 

Fixing  Natron,  24 

Foreshortening  in  Perspective,  148 

Fritzsch,  209 

GAFFIELD,  270 

Gal vano- plastic,  227 

Genre,  Tableaux  de,  126 

Glass,  its  Combination  with  Light, 

269 
Glass  Painting  and  Photography, 

259 
Globes,  Abnormal  rendering  of,  by 

Photography,  124 
Greens,  209,  213 
Groups,  Pictures  of,  153 
Gun  Cotton,  34 

HALF-TONES  reproduced  by  Helio- 
graphy,  230;  by  Photo-litho- 
graphy, 252;  in  Photo-reliefs, 
234 

Heliography  with  Asphalt,  10 ;  with 
Salts  of  Silver,  with  Salts  of 
Chromium,  214 

Heliopictor,  201 

Herschel,  24 

Hornsilver,  4 

IMAGES,  Optical,  by  Refraction,  83 

Accuracy  of  Photographic, 

120 

Their   dependence  on  the 

Artist,  the  Apparatus,  and 
the  Original ;  their  Differ- 
ence and  Keality,  120-133 
Infernal  Stone,  5 

Instruments,  Treatment  of  Scien- 
tific, by  Photography,  200 
Iodine,  17,  109 

Iodide  and  Bromide  of  Silver,  113 
Iron,  Salts   of,  Photography  with, 
265 

JOUBERT,  257 

Jurisprudence  and  Photography,  217 


K  AUSTEN,  16 
Kellner,  252 
Kirchoff,  63 

LANDSCAPE  and  Photography,  159 

Land- Survey  ing  Photography,  166 

Lantern,  Magic,  93,  283 

Leaf  Prints,  26 

Leisner,  258 

Lenses,  87 

Libraries,  Use  of  Microscopic  Pho- 
tographs in,  210 

Light,  as   a   Chemically-Operative 
Agent,  55 

Its  Nature,  56 

Chemical  Effects  of,  105 

Electric,  71 

Lime,  70 

Magnesium,  69 

Sky,  77 

Sun,  75 

Eefraction  of,  83 

-Pictures,  22 


-Paus  Paper,  23 


Lithography,  248 

MAGIC  Cigars,  214 
Magic  Photography,  215 
Magnesium  Light,  69 
Magnifying  Apparatus,  156 
Medical  Research   and   Photogra- 
phy, 202 
Mercury,  17,  112 
Meydenbauer,  171 
Micro-photographs,  208 
Microscopes,  207 
Microscopic  Photographs,  209 
Minerals,  Change  of,  by  Light,  270 
Mitscherlich,  122 
Morse,  17 
Moser,  16,  103 

NATRON,  Hypo-sulphite  of,  44 
Negatives,  28,  30 
Negative  Process,  37 
Negative  Retouche,  49 
Neumeyer,  201 
Niepce  Nicophore,  9 


INDEX. 


Niepce  de  St.  Victor,  33,  274 


OBERNETTER,  245,  257,  268 

Oidtmann,  258 

Osborne,  251 

Ozone,  Formation  of,  by  Light,  108 

PALING  of  Photographs,  55 

Panoramic  Apparatus,  165 

Pantograph,  231 

Paper,  Durable,  Sensitive,  30 

Paper  Negatives,  28,  30 

Paper  Photography,  23 

Parallactic  Exposition,  172 

Patterns,  Photographic,  220 

Paus  Process,  24 

Pea- Sausage,  264 

Perspective,  and  its  Influence  on 
Photography,  134 

Perspective  Foreshortenings,  135 

Petzval,  19 

Phosphorus,  Sensitive  to  Light,  108 

Photographs  from  Photographs,  158 

Photography  Development  of,  Mo- 
dern, 31 

Photo-lithography,  248 

Photo-reliefs,  230 

Photo -zincography,  250 

Photometer,  262 

Physical  Effects  of  Light,  1 

Pigment  Press,  239 

Plates,  Cleaning  of,  39 

Poitevin,  239,  243,  251,  257,  275 

Ponton,  224 

Porta,  8 

Portraits,  154 

Portrait  Objectives,  19 

Portrait  Photography,  149 

Choice  of  Dress  for,  153 

Influence  of  Weather  on, 

155 

•  Influence  of  the  Size  of 

the  Picture  in,  156 

Positive,  29 

Potash  and  Glue,  225 

Prisms,  62 

Protuberances,  182 

Pretsch,  227 


Protection  or* 
Pyrogallic  Acid,  35 
Pyro-photography    with    Salts     of 
Silver,  212 

QUICKSILVER  (see  Mercury) 

REFRACTION  of  Light,  83 

Relief    (High  and  Low)    Printing, 

235 

Beliefs,  234 
Belief  Press,  237 
Retouche  Negative,  49 
Rutherford,  187,  190,  191 

SACHSE,  16 

Sand-blowing  Process,  260 

Scamoni,  215,  230 

Shades,  135 

Schonbein,  34 

Schools,  277 

Seebeck,  273 

Sensibilisators,  115 

Silver,  Hypo-bromide  of,  111 

Hypo -chloride  of,  111 

Contents   of    Photographs, 


119 


Hypo -iodide  of,  111 
-  Salts,  Effects  of  Light,  on, 


109 
Use    of,    in    Photography, 

119 

Solar  Camera,  95 
Sun  and  Sun-spots,  taking  of  the, 

186 
Sunlight,  Chemical  Effect  of,  77 

Reflected,  74 

Clearness  of,  75 

Spectrum,  62 

Spectral  Analysis,  63 

Spectral  Lines,  63,  194 

Steel  Engraving,  Photographic,  226 

Standpoint,  Effect  of  the,  146 

Stone,  199 

Stereoscope,  American,  102 

Stereoscopic  Landscapes,  163 

Stars,  taking  of,  188 

Strengthening,  47 


288 


INDEX. 


TALBOT,  Fox,  23,  225 
Talbot-type,  23 

Paper  Photographs;  23 

Talbot,  R.,  30 
Tent  Photography,  161 
Tessie  de  Mothay,  243 
Thermometers  Registered  by  Pho- 
tography, 200 
Tilghmann,  260 
Tones  and  Colours,  59 
Tones  of  Paper  Pictures,  52 
Topaz,  its  Sensitiveness  to  Light, 

271 
Transparent  Pictures  on  Glass,  163 


UNDULATIONS  of  Light,  57 
Uranium  Salts,  267 


VARNISH,  46 

Yenus,  Transit  of,  197 

Visite,  Cartes  de  (see  Cartes) 

WAX,  5 

Warren  de  la  Rue,  174,  194 

Wedgwood,  5 

Wheatstone,  98 

White  of  Egg  (see  Egg) 

Willis,  247 

Woodbury,  237 

YELLOW  Light,  Effect  of,  on  Plants. 

80 
On  Salts,  42,  63 

ZENCKER,  275 
Zincography,  250 


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